Webasto Furnace: “F09”
warning RV furnace fault codes
What it means
Glow plug / ignition unit fault - the glow plug (or electronic ignition unit) circuit shows a short or open circuit, so ignition cannot be performed reliably.
Most likely causes
- Open circuit (break) in the glow plug or its wiring common
- Short circuit in the glow plug or ignition spark generator common
- Worn-out glow plug at end of life less common
Safe checks you can do yourself
- Inspect the glow plug wiring and connector for damage or corrosion.
- Confirm the connector is seated.
How to reset / clear F09
- Repair any wiring fault, then switch the heater off and on.
- Clear a lock-out with the fuse-pull reset if the heater remains locked.
When to call a technician
If wiring checks out, the glow plug should be tested and replaced 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 "F09" mean?
Glow plug / ignition unit fault - the glow plug (or electronic ignition unit) circuit shows a short or open circuit, so ignition cannot be performed reliably.
How do I reset Webasto Furnace F09?
Repair any wiring fault, then switch the heater off and on. Clear a lock-out with the fuse-pull reset if the heater remains locked.
What is the most common cause of F09 on a Webasto Furnace?
The most likely cause is open circuit (break) in the glow plug or its wiring.
Related · Webasto furnace
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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… F04
Premature flame detection - the flame/exhaust temperature mo… F07
Fuel pump fault - the dosing/fuel pump circuit shows an open…
Sources
- Air Top Evo 40 / Air Top Evo 55 - Fault Codes / Troubleshooting (Fig. 503) — Section 5.4, Fault code F 09 (PC codes 19 short / 99 open) (Butlertechnik / Webasto)
- Webasto Air Top 2000 STC Fault Error Codes — Section 5.5, F09 Glow plug / ignition unit interruption (Butlertechnik / Webasto)
Note: EVO labels this 'Glow plug defective' (F09); the 2000 STC labels the FXX output 'Glow plug / electronic ignition unit interruption' and also logs hex 19/99/0A/8A variants.
● Last verified 2026-06-29 · compiled from manufacturer documentation. Codes can vary by model year — always cross-check your unit’s manual.