Gledhill GB35C Fault codes & diagnostics

63 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 35 kW 2006 onwards Discontinued
GC Number 47-317-01
Download the Gledhill GB35C manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
2 Emergency 61 High
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-317-01 GB35C AGB5035 2006–present
Just want the most common Gledhill GB35C faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 63 documented codes

Tap any card for details

LED / flash-pattern codes
Flashing dot D2

Your boiler has detected a general ignition or operational failure and has locked itself out for safety.

High DIY-safe £100-280

What to check first

  1. Locate the 'set/reset' button (labelled S1) on the control panel.
  2. Press and release the S1 button to clear the fault.
  3. Observe if the boiler fires up and the flashing dot disappears.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • PC B Control Board

Related codes

Full explanation for Flashing dot D2
FLASHING P

Your boiler has detected that the water pressure is too low, causing it to stop running to protect the system.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath the boiler.
  2. Slowly open the tap or taps on the filling loop to let water into the system.
  3. Watch the pressure gauge and close the taps once the needle reaches between 1 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for FLASHING P
Digital codes
10

Your boiler's computer memory has become corrupted, preventing it from understanding its own internal settings and operating instructions.

High Engineer only £250-450

Parts commonly replaced

  • PCB (Printed Circuit Board)
  • Control unit
Full explanation for 10
11

Your boiler has detected that the water in your central heating system is getting too hot and has automatically shut down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • NTC temperature sensor
  • Pump
  • Diverter valve

Related codes

Full explanation for 11
12

Your boiler's computer memory has encountered a digital error and cannot correctly process its settings, preventing it from running safely.

High Engineer only £200-400

Parts commonly replaced

  • Printed Circuit Board (PCB)
Full explanation for 12
13

Your boiler's computer is experiencing an internal processing error, meaning it has lost track of its current operating status and needs a professional to reset its logic.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB
  • Wiring harness

Related codes

Full explanation for 13
14

Your boiler's computer memory has encountered a serious internal error, preventing it from operating safely or correctly.

High Engineer only £250-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for 14
15

Your boiler has detected that it is getting too hot and has automatically switched itself off to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • Heat exchanger
  • Diverter valve

Related codes

Full explanation for 15
16

Your boiler has detected an internal communication error between its main control components, causing it to shut down for safety.

High Engineer only £150-300

Parts commonly replaced

  • PCB
  • Wiring harness

Related codes

Full explanation for 16
17

Your boiler is struggling to accurately measure the difference in temperature between the water flow and return, preventing it from heating your home safely.

High Engineer only £150-250

Parts commonly replaced

  • Temperature sensors (NTC thermistors)
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 17
Showing 11–20 of 63
18

Your boiler has detected that the water inside the system has become dangerously hot and has shut itself down to prevent damage or overheating.

High Engineer only £150-300

Parts commonly replaced

  • NTC thermistor
  • Primary circulation pump
  • Heat exchanger
  • PCB

Related codes

Full explanation for 18
19

Your boiler's computer has detected an internal processing error, meaning it has safely shut itself down to prevent potential damage.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB

Related codes

Full explanation for 19
20

Your boiler is struggling to turn off the flame as quickly as it should after a heating cycle, which is a safety concern that prevents it from restarting.

Emergency Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Printed circuit board (PCB)
  • Flame detection electrode

Related codes

Full explanation for 20
21

Your boiler is failing to ignite or stay lit, meaning it cannot safely produce heat or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe

Related codes

Full explanation for 21
22

Your boiler has detected an internal communication failure between its main control components and has shut down as a safety precaution.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
  • Control interface

Related codes

Full explanation for 22
23

Your boiler's electronic control system has detected a communication failure, meaning it cannot safely command the burner to ignite.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 23
24

Your boiler has tried to light the burner multiple times but has failed, so it has safely shut itself down to prevent a build-up of unburnt gas.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe

Related codes

Full explanation for 24
25

Your boiler has detected an internal communication error regarding its ability to safely confirm a flame, which prevents it from operating correctly.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
  • Flame rectification sensor

Related codes

Full explanation for 25
26

Your boiler has detected a communication error within its internal computer system, meaning it cannot process instructions correctly and has stopped working to prevent further damage.

High Engineer only £150-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for 26
28

Your boiler has detected an internal communication error between its main control components, causing it to shut down for safety.

High Engineer only £150-300

Parts commonly replaced

  • Main printed circuit board (PCB)
  • Wiring harness
  • Control interface

Related codes

Full explanation for 28
Showing 21–30 of 63
29

Your boiler has detected an internal communication error with its main control board, meaning it cannot process instructions correctly and has stopped working for your safety.

High Engineer only £250-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 29
30

Your boiler's internal computer is experiencing a communication error and cannot process its instructions correctly.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 30
31

Your boiler's internal electronic system is struggling to read a reference voltage correctly, which is preventing the appliance from starting safely.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 31
33

Your boiler has detected that the water pressure in your heating system is currently too low for it to operate safely.

High DIY-safe £0-£120

What to check first

  1. Locate the external filling loop, which is a flexible silver hose connected between two pipes under your boiler.
  2. Open the valves at both ends of the filling loop to allow cold mains water to enter the system.
  3. Watch the pressure gauge on the front of the boiler until the needle reaches the green zone, typically between 1.0 and 1.5 bar.
  4. Close both valves firmly and ensure the flexible hose remains attached or capped as required.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor

Related codes

Full explanation for 33
34

Your boiler has detected that the water flow through the heating system is too high or blocked, causing it to stop operating to protect itself.

High Engineer only £150-300

Parts commonly replaced

  • Flow sensor
  • Main circuit board (PCB)
  • Pump

Related codes

Full explanation for 34
36

Your boiler is failing to light the burner or keep it running, which means it cannot produce hot water or heating.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Flame rectification probe
  • Gas valve
  • PCB

Related codes

Full explanation for 36
38

Your boiler is failing to detect that the airflow system is operating correctly, preventing it from safely igniting.

High Engineer only £150-300

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venturi

Related codes

Full explanation for 38
39

Your boiler's safety sensor is not detecting the air flow required to start the system, so it has safely shut itself down to prevent ignition.

High Engineer only £150-300

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Pressure sensing tubes

Related codes

Full explanation for 39
44

Your boiler is struggling to process the electricity supply correctly, which has caused it to shut down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 44
45

Your boiler is struggling to detect a consistent electricity supply, which is preventing it from starting up correctly.

High Engineer only £120-250

Parts commonly replaced

  • Printed Circuit Board (PCB)
Full explanation for 45
Showing 31–40 of 63
46

Your boiler has detected an electrical safety issue with its grounding, which means it has safely shut itself down to prevent a risk of electric shock.

Emergency Engineer only £120-250

Parts commonly replaced

  • Wiring harness
  • PCB
  • Earth bonding connection

Related codes

Full explanation for 46
47

Your boiler's internal computer is struggling to communicate with its main control unit, causing the system to stop functioning correctly.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Communication interface

Related codes

Full explanation for 47
48

Your boiler's computer is confused because it has not been told which specific model it is, preventing it from starting up correctly.

High Engineer only £120-250

Parts commonly replaced

  • Main control board
  • Configuration plug

Related codes

Full explanation for 48
49

Your boiler's control system is struggling to read data from multiple sensors, so it has shut down as a precaution to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • NTC sensors
  • Wiring harness
  • Main control PCB

Related codes

Full explanation for 49
50

Your boiler's temperature sensor has disconnected or failed, so the system has shut down to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for 50
51

Your boiler has lost its connection to an internal temperature sensor, preventing it from knowing how hot the water is.

High Engineer only £120-220

Parts commonly replaced

  • NTC Sensor
  • Wiring harness

Related codes

Full explanation for 51
53

Your boiler's temperature sensor has a faulty connection, which prevents it from reading the water temperature correctly and stops the system from heating up.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for 53
54

Your boiler has lost the connection to one of its internal temperature sensors, meaning it cannot correctly measure heat levels and has shut down as a precaution.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for 54
55

Your boiler has lost its connection to an internal temperature sensor, causing it to stop operating to ensure safety.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for 55
56

Your boiler has lost its connection to an internal temperature sensor, which is preventing it from safely heating your water.

High Engineer only £150-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for 56
Showing 41–50 of 63
57

Your boiler has detected a faulty connection to a temperature sensor, which is preventing it from safely heating your water or home.

High Engineer only £120-220

Parts commonly replaced

  • Domestic Hot Water (DHW) sensor
  • NTC thermistor
  • Wiring harness

Related codes

Full explanation for 57
58

Your boiler has detected an electrical fault with one of its internal temperature sensors, meaning it cannot accurately monitor heat levels and has shut down as a precaution.

High Engineer only £150-250

Parts commonly replaced

  • NTC temperature sensor
  • Wiring harness
  • Printed Circuit Board

Related codes

Full explanation for 58
59

Your boiler has detected a faulty electrical signal from one of its internal temperature sensors, causing it to shut down for safety.

High Engineer only £150-250

Parts commonly replaced

  • NTC Sensor
  • Wiring harness

Related codes

Full explanation for 59
60

Your boiler has detected an electrical fault with one of its internal temperature sensors, which is preventing it from heating your home or water reliably.

High Engineer only £150-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for 60
61

Your boiler has detected an electrical fault in a temperature sensor, which has caused it to shut down to prevent incorrect operation.

High Engineer only £120-200

Parts commonly replaced

  • Domestic Hot Water (DHW) sensor
  • NTC thermistor

Related codes

Full explanation for 61
63

Your boiler has detected an electrical fault with one of its internal temperature sensors, preventing it from safely monitoring heat levels, so it has shut down to protect itself.

High Engineer only £150-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for 63
64

Your boiler has detected a fault with one of its internal temperature sensors, which means it cannot accurately monitor heat levels and has shut down as a safety precaution.

High Engineer only £150-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for 64
65

Your boiler has detected a faulty reading from an internal temperature sensor, which has caused it to stop working to ensure your safety.

High Engineer only £150-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for 65
66

Your boiler's reset button is currently stuck or malfunctioning, preventing it from restarting correctly.

High Engineer only £120-250

Parts commonly replaced

  • User Interface PCB
  • Control Panel Assembly

Related codes

Full explanation for 66
C0

Your boiler’s internal computer chip has encountered a memory reading error, preventing it from processing the instructions needed to function.

High Engineer only £280-450

Parts commonly replaced

  • Main Control PCB
  • Display Interface Module

Related codes

Full explanation for C0
Showing 51–60 of 63
C1

Your boiler has tried to light several times but has failed to create a flame, meaning your heating and hot water will not work.

High DIY-safe £120-350

What to check first

  1. Check that your gas emergency control valve is in the 'on' position
  2. If you have a pre-payment meter, ensure you have credit and that the meter hasn't locked out
  3. Press the Reset button on the boiler control panel for 3 seconds

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing probe
  • Spark generator

Related codes

Full explanation for C1
C6

Your boiler has detected an internal electrical safety failure on its main control board and has shut down to prevent damage.

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Safety Relay

Related codes

Full explanation for C6
C8

Your boiler has stopped working because the fan that clears exhaust gases from the system is not spinning at the correct speed.

High Engineer only £280-450

Parts commonly replaced

  • Fan Assembly
  • Main PCB
  • Wiring Harness

Related codes

Full explanation for C8
C13

Your boiler has experienced an internal software or communication error and has stopped working to protect itself.

High DIY-safe £100-£350

What to check first

  1. Turn the boiler's electrical power switch off
  2. Wait for approximately 30 seconds
  3. Turn the power back on to see if the system clears the internal error

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Display Interface

Related codes

Full explanation for C13
c33

Your boiler has detected that the water temperature returning to the unit is too low, causing it to shut down for safety.

High Engineer only £120-350

Parts commonly replaced

  • Return thermistor
  • Circulation pump
  • Main PCB

Related codes

Full explanation for c33
c53

Your boiler has stopped working because one of its internal temperature sensors has become disconnected or developed a fault.

High Engineer only £120-£200

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for c53
D1

Your boiler has experienced a lockout, meaning it has switched itself off for safety after failing to fire correctly.

High DIY-safe £90-250

What to check first

  1. Locate the reset button (labeled S1) on the control panel.
  2. Press and hold the reset button for a few seconds to see if the boiler restarts.
  3. If the boiler locks out again immediately, do not keep resetting it and call a professional.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Probe

Related codes

Full explanation for D1
En

Your boiler has detected that one of its internal sensors is sending an incorrect signal or has failed, which is preventing the system from operating correctly.

High Engineer only £120-£240

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness
  • Control Board

Related codes

Full explanation for En
Ignition lockout

Your boiler has tried to start three times but hasn't been able to light the flame, so it has shut down for safety.

High DIY-safe £100-£250

What to check first

  1. Check that your gas pre-payment meter (if you have one) has credit.
  2. Ensure your external gas emergency control valve is turned to the 'on' position.
  3. Press the central button (S1) beneath the boiler display to attempt a reset.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Spark generator

Related codes

Full explanation for Ignition lockout
Lockout error

Your boiler has stopped working safely and needs a simple restart to clear a temporary system error.

High DIY-safe £0-120

What to check first

  1. Locate the 'Reset' button (labeled S1) on the boiler control panel.
  2. Press and hold the Reset button for 2-3 seconds.
  3. Wait for the boiler to complete its startup sequence and check if the error light stops flashing.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Probe

Related codes

Full explanation for Lockout error
Showing 61–63 of 63
Low pressure safety trip

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

High DIY-safe £0-120

What to check first

  1. Check the pressure gauge; it is likely below 1.0 bar.
  2. Locate the filling loop (braided silver hose) usually under the boiler.
  3. Open the valves slowly to top up the water until the gauge reaches 1.5 bar.
  4. Close the valves tightly and press the S1 button on the boiler to reset the system.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for Low pressure safety trip
Overheat safety trip

Your boiler has switched itself off to prevent it from getting too hot and causing internal damage.

High DIY-safe £100-250

What to check first

  1. Wait for the boiler to cool down completely
  2. Locate the S1 reset button on the control panel
  3. Press and hold the button to clear the fault and restart the unit

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Heat Exchanger (de-scale)

Related codes

Full explanation for Overheat safety trip
P

Your boiler has detected that the water pressure in the system is too low to run safely and has stopped working to protect itself.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.
  2. Slowly open the valve or valves on the filling loop until you hear water flowing.
  3. Watch the pressure gauge and close the valves tightly once the needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for P
Status & diagnostic codes

6 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

FLASHING F Status Your boiler has detected very cold temperatures and has automatically turned on its high-level frost protection mode to prevent the internal pipes from freezing. Blocking error Status Your boiler has paused operation because it has detected a temporary issue that is currently preventing it from running safely or efficiently. E1 Diagnostic Your boiler has detected an electrical fault with one of its internal sensors or its wiring, which means it cannot accurately monitor temperatures and has stopped working for safety. E3 Diagnostic Your boiler has stopped working because it has detected that the water inside has become dangerously hot. F Status Your boiler has detected that the water temperature is too low and has automatically turned on its internal heater to prevent the pipes from freezing. P1.6 Diagnostic Your boiler is notifying you that the water pressure in the system is currently at 1.6 bar, which is within the normal operating range.