Johnson Starley C26d Fault codes & diagnostics

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

Combi Natural Gas 26 kW 2007-2011 Discontinued
GC Number 42-417-98

Replaced by: Quantec HR28C

Download the Johnson Starley C26d manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 12 High 1 Medium
Just want the most common Johnson Starley C26d faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 14 documented codes

Tap any card for details

13

Your boiler has locked itself out because too many remote reset attempts were made in a short space of time.

Medium DIY-safe £0-120

What to check first

  1. Locate the main power switch (fused spur) for the boiler.
  2. Turn the power switch to the 'Off' position.
  3. Wait for 30 seconds, then turn the power back 'On' to clear the lockout.

Parts commonly replaced

  • No parts usually required
  • External controls/thermostat
Full explanation for 13
21

Your boiler's main internal computer has developed a communication fault and can no longer control the heating safely.

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB (ESYS Control Board)
Full explanation for 21
25

Your boiler's internal computer chips are failing to communicate because their software versions no longer match.

High Engineer only £280-450

Parts commonly replaced

  • ESYS Control Board
  • PCB Display Module

Related codes

Full explanation for 25
26

Your boiler is struggling to keep its burner lit and has shut down for safety after failing to detect a steady flame several times.

High Engineer only £120-350

Parts commonly replaced

  • Flame sensing electrode
  • Ignition control board
  • Gas valve
  • Fan

Related codes

Full explanation for 26
30

Your boiler has stopped working because it thinks the air in the heating ducts is overheating due to a faulty electrical sensor.

High Engineer only £120-£220

Parts commonly replaced

  • Duct air temperature sensor (thermistor)
  • Wiring harness

Related codes

Full explanation for 30
31

Your boiler has stopped working because it cannot detect the temperature of the air inside the heating system.

High Engineer only £120-220

Parts commonly replaced

  • Duct air temperature sensor (thermistor)
  • Wiring harness

Related codes

Full explanation for 31
34

Your boiler is not receiving enough electrical power from your home's supply to operate safely.

High Engineer only £90-250

Parts commonly replaced

  • Electrical wiring
  • Printed Circuit Board (PCB)
  • Voltage regulator
Full explanation for 34
43

Your boiler has stopped working because its internal temperature sensor has developed an electrical fault and is unable to read the air temperature correctly.

High Engineer only £120-190

Parts commonly replaced

  • Return air temperature sensor (thermistor)
  • Wiring harness
Full explanation for 43
44

Your boiler has lost its connection to the sensor that monitors the temperature of the air returning to the unit, causing it to stop heating as a safety precaution.

High Engineer only £120-220

Parts commonly replaced

  • Return air temperature sensor
  • Wiring harness
  • PCB

Related codes

Full explanation for 44
45

Your boiler has detected an electrical fault with the sensor that monitors exhaust gases, causing the system to shut down for safety.

High Engineer only £120-220

Parts commonly replaced

  • Flue gas temperature sensor (NTC)
  • Wiring harness
  • Control board (PCB)

Related codes

Full explanation for 45
46

Your boiler has stopped working because it cannot detect the temperature of the exhaust gases, causing it to shut down for safety.

High Engineer only £120-£210

Parts commonly replaced

  • Flue Gas Temperature Sensor (NTC)
  • Wiring Harness Connector

Related codes

Full explanation for 46
57

Your boiler is struggling to breathe properly, meaning it cannot safely vent exhaust gases, so it has shut down to prevent a potential safety hazard.

Emergency Engineer only £150-350

Parts commonly replaced

  • Flue assembly
  • Fan
  • Air pressure switch
  • Venturi

Related codes

Full explanation for 57
99

Your boiler's main control panel is unable to communicate with the internal computer system, causing the boiler to stop working for safety reasons.

High Engineer only £150-300

Parts commonly replaced

  • Interface cable
  • Main PCB
  • Display module

Related codes

Full explanation for 99
E18

Your boiler has stopped working because the waste water produced during heating cannot drain away correctly, usually due to a blocked or faulty pump.

High DIY-safe £120-220

What to check first

  1. Check the external white plastic drainage pipe for any signs of freezing or blockages.
  2. Ensure any flexible hoses connected to the condensate pump are not kinked or trapped.
  3. If the outlet pipe is frozen, gently pour warm (not boiling) water over the pipe to thaw it.

Parts commonly replaced

  • Condensate pump
  • Float switch sensor
  • Condensate discharge pipework
Full explanation for E18