Webasto Furnace: “F04”
warning RV furnace fault codes
What it means
Premature flame detection - the flame/exhaust temperature monitor sensed a flame before combustion was supposed to begin, so the heater aborts as a safety measure.
Most likely causes
- Exhaust/flame temperature sensor still reading hot residual heat from a recent cycle common
- Defective or out-of-tolerance exhaust temperature sensor common
- On 2000 STC: residual fuel/air or fuel-system fault causing early combustion signal less common
Safe checks you can do yourself
- Let the heater fully cool, then retry - a hot restart can trigger this.
- Check the combustion air intake and exhaust for obstructions.
- On 2000 STC, verify the fuel system and fuel pump are functioning.
How to reset / clear F04
- Allow the unit to cool down completely.
- Switch off and on again to retry the start sequence.
When to call a technician
If the code persists after cooling, the exhaust temperature sensor likely needs checking or replacement by a technician.
Applies to
Air Top EVO 40 · Air Top EVO 40 B (petrol) · Air Top EVO 40 D (diesel) · Air Top EVO 55 · Air Top 2000 STC
FAQ
What does Webasto Furnace code "F04" mean?
Premature flame detection - the flame/exhaust temperature monitor sensed a flame before combustion was supposed to begin, so the heater aborts as a safety measure.
How do I reset Webasto Furnace F04?
Allow the unit to cool down completely. Switch off and on again to retry the start sequence.
What is the most common cause of F04 on a Webasto Furnace?
The most likely cause is exhaust/flame temperature sensor still reading hot residual heat from a recent cycle.
Related · Webasto furnace
The heater's control unit (ECU) has an internal fault, the h… F01
No start - the heater attempted ignition and, even after a r… F02
Flame abort / flame failure - a flame was established but we… F03
Supply voltage out of range - the heater's operating voltage… F07
Fuel pump fault - the dosing/fuel pump circuit shows an open… F08
Drive/combustion-air motor fault - the fan motor has an open…
Sources
- Air Top Evo 40 / Air Top Evo 55 - Fault Codes / Troubleshooting (Fig. 503) — Section 5.4, Fault code F 04 (PC code 05) (Butlertechnik / Webasto)
- Webasto Air Top 2000 STC Fault Error Codes — Section 5.5, F04 Flame detected prior to combustion (Butlertechnik / Webasto)
Note: EVO uses the exhaust temperature sensor for flame detection; the 2000 STC additionally lists fuel-system checks for this code.
● Last verified 2026-06-29 · compiled from manufacturer documentation. Codes can vary by model year — always cross-check your unit’s manual.