Common problems guide

The most common Vaillant Home Regular 12 problems

The faults most likely to send a Vaillant Home Regular 12 into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.

The 6 most common faults

01

F.00 Engineer

Fault: Flow temperature sensor - NTC plug not plugged in or has come loose, multiple plug on the PCB not plugged in correctly, interruption in cable harness, NTC sensor defective

Full guide for F.00

02

F.01 Engineer

Fault: Return temperature sensor - NTC plug not plugged in or has come loose, multiple plug on the PCB not plugged in correctly, interruption in cable harness, NTC sensor defective

Full guide for F.01

03

F.10 Engineer

Short circuit: Flow temperature sensor - NTC sensor defective, short circuit in the cable harness, cable/casing

Full guide for F.10

04

F.11 Engineer

Short circuit: Return temperature sensor - NTC sensor defective, short circuit in the cable harness, cable/casing

Full guide for F.11

05

F.13 Engineer

Short circuit: Domestic hot water cylinder temperature sensor - NTC sensor defective, short circuit in the cable harness, cable/casing

Full guide for F.13

06

F.20 Engineer

Safety switch-off: Overheating temperature reached - Incorrect earth connection between cable harness and product, flow or return NTC defective (loose connection), black discharge via ignition cable, ignition plug or ignition electrode

Full guide for F.20

Call a Gas Safe engineer if…

You can smell gas or see signs of a leak
The Vaillant shows an Emergency or High severity code
The boiler keeps locking out after repeated resets
You've tried the DIY checks and the fault hasn't cleared
There's visible water leaking from the boiler
The flame is yellow or orange instead of blue

See the full 33-code list for the Vaillant Home Regular 12