F.042 on a Glow-worm GeniaAir: The coding resistor (in the wiring harness) or the gas group resistor (on the PCB, if available) is invalid
Short answer
On a Glow-worm heat pump, F.042 means: The coding resistor (in the wiring harness) or the gas group resistor (on the PCB, if available) is invalid. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: The coding resistor (in the wiring harness) or the gas group resistor (on the PCB, if available) is invalid.
Part involved: The coding resistor (in the wiring harness) or the gas group resistor (on the PC.
Likely causes:
- Interruption in the wiring harness to the fan; incorrect wiring harness between PCB and gas valve assembly/heat cell; heat cell coding resistor not detected (with F.070); fan coding resistor defective (table text appears shared with Glow-worm boilers).
- Glow-worm trade page: Coding resistor fault Coding resistor damaged or not set, damage to coding resistor wiring harness.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Check the heat pump supply breaker/isolator (and back-up heater breaker, if separate) at the consumer unit is on; if a breaker has tripped more than once, leave it off and call an engineer - do not open any casing. Glow-worm's MiPro Sense operating instructions say always contact your competent person in the event of a fault message; until they arrive you may select a limp-home (back-up heater) setting on the control, which is expensive to run.
What the engineer will check
- Check wiring harness and plug connections; check harness article number; check the coding resistor (PCB plug X25, contact 11/12); check/replace fan.
Who to ask for: Heat pump/controls installer familiar with Vaillant eBUS controls.
Certification: MCS installer certification; Vaillant product training recommended.
What you might notice
Sources
- GeniaAir Appliance Interface installation and maintenance instructions ↗ · 8000043014_01 · page HPIM 7 8000043014_01 p.31 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026