Webasto F03: supply voltage too low or too high

What F03 means

The code does not distinguish "too little" from "too much": both are in one entry, and the difference can be found by workshop diagnostics. For the Air Top 2000 ST the factory thresholds are:

For other models look in the manual for your model. The factory advice in both cases is the same: charge the battery.

Thermo Top Evo

On the Thermo Top Evo F03 is also about the on-board voltage, too low or raised. The manual advises charging the vehicle battery and then switching the heater off and on again.

Can you drive

A heater is not an engine, and F03 by itself does not forbid driving. But the code points to a power problem, and a discharged battery may fail to start the vehicle in the morning. Charge or check it before the next overnight stop.

Order of actions

1. Inspect the heater fuses and connectors, where the factory instruction starts. Look for corrosion, loose contacts, poor crimps, corroded wires and fuses.
2. Inspect the battery terminals: corrosion, loose nuts.
3. Charge the battery.
4. Switch the heater off for at least 2 seconds and on again.

When to see a workshop

• F03 did not go away after charging the battery and checking contacts: the instruction advises a check at a Webasto-trained service;
• F03 keeps coming back with a charged battery.

Questions and answers

Is F03 on a Webasto a flame-out?

No. Flame-out on the timer shows as F02, while F03 is the on-board voltage.

At what voltage is F03 set on the Air Top 2000 ST?

Below 10.5 V (21 V on a 24 V system) for more than 20 seconds or above 16 V (32 V) for more than 6 seconds.

What should I do first?

Inspect the fuses and connectors, then the battery terminals, charge the battery and switch the heater off and on with a pause of at least 2 seconds.

Why does the code stay with a charged battery?

Then workshop diagnostics are needed: the cause may be the wiring or the power connectors rather than the battery itself.

Code pages on the site

• Webasto F03 - code page
• F02 - flame-out
• F01 - no start
• All Webasto codes