Webasto Furnace: “F03”
warning RV furnace fault codes
What it means
Supply voltage out of range - the heater's operating voltage was either too low or too high for a sustained period, so it shut down to protect itself and the battery system.
Most likely causes
- Undervoltage: weak/discharged battery, poor connections, corroded terminals (below 10.5 V on 12 V / 20.5 V on 24 V for >20 s) most likely
- Overvoltage: alternator/voltage regulator overcharging (above 16 V on 12 V / 32 V on 24 V for >6 s) common
- High resistance in supply wiring, fuses, or battery terminals common
Safe checks you can do yourself
- Measure battery voltage and check it under load.
- Inspect and clean battery terminals, fuses, and the heater's power/ground connections.
- On overvoltage, have the charging system/voltage regulator checked.
How to reset / clear F03
- Restore the battery to a healthy charge or correct the charging fault.
- Switch the heater off and on again once voltage is back in range.
When to call a technician
If voltage is correct at the battery but the heater still reports F03, or if overvoltage points to a charging-system fault, have it diagnosed professionally.
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 "F03" mean?
Supply voltage out of range - the heater's operating voltage was either too low or too high for a sustained period, so it shut down to protect itself and the battery system.
How do I reset Webasto Furnace F03?
Restore the battery to a healthy charge or correct the charging fault. Switch the heater off and on again once voltage is back in range.
What is the most common cause of F03 on a Webasto Furnace?
The most likely cause is undervoltage: weak/discharged battery, poor connections, corroded terminals (below 10.5 v on 12 v / 20.5 v on 24 v for >20 s).
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Sources
- Air Top Evo 40 / Air Top Evo 55 - Fault Codes / Troubleshooting (Fig. 503) — Section 5.4, Fault code F 03 (PC codes 84 undervoltage / 04 overvoltage) (Butlertechnik / Webasto)
- Webasto Air Top 2000 STC Fault Error Codes — Section 5.5, F03 Supply voltage too high (hex 04 over / 84 under) (Butlertechnik / Webasto)
Note: On the EVO, F03 covers both under- and over-voltage; on the 2000 STC the FXX label reads 'Supply voltage too high' (overvoltage) while low voltage appears as hex 84. Voltage thresholds shown are for 12 V / 24 V systems respectively.
● Last verified 2026-06-29 · compiled from manufacturer documentation. Codes can vary by model year — always cross-check your unit’s manual.