U0325: Software Incompatibility With Auxiliary Heater Control Module
What to do
The auxiliary heater will not run properly, and there is one safety point that matters more than the inconvenience.
These heaters burn fuel and have their own exhaust, so they produce carbon monoxide. Never run one in a garage or any enclosed space, and if it starts when it should not, or refuses to shut down, have it isolated rather than living with it. If this is a motorhome or anything people sleep in, treat that as urgent.
Otherwise nothing is broken — two computers are running software that does not belong together, which is a programming job.
If your heater was fitted after the car or van was built, go back to the installer rather than the main dealer. They fitted that module and hold its configuration, and a dealer updating the factory software again will not fix a mismatch on the aftermarket side.
And here is the commonest way this happens, with nobody at fault. The vehicle went to a dealer, the factory software was updated under a service action, and the heater's computer was left on the version it was installed with. If it has been to a dealer recently, mention it — that is very likely the whole story.
Expect cold starts and a cold cabin until it is sorted, which on a diesel also means more soot into the particulate filter.
What this code means
A module is running software that does not belong with the auxiliary heater control module's. See U0300 for the family and U0166 for what an auxiliary heater is and why it matters beyond comfort.
This code has a dominant cause that most of the family does not, and it is the same one as U0309: a retrofit.
Fuel-burning parking heaters are very often fitted after the vehicle was built — by a specialist, on a van, a motorhome, a caravan conversion or a cold-climate car — and the heater's own module has to agree with the vehicle about when it may run, what the coolant temperature is, whether the fuel level permits it and how the timer and remote are handled. That agreement is software on both sides.
So the commonest route is a vehicle whose factory software was updated while the retrofit's was not — a dealer visit, a recall, a module replacement — with nobody at fault and the two halves simply no longer matching. The installer is usually the right first call, since a dealer updating the factory side again cannot resolve a mismatch on the aftermarket side.
One safety point belongs on any page about a fuel-burning heater and does not soften for a configuration fault. These units have their own exhaust and produce carbon monoxide. A heater whose control agreement is uncertain must not be left to run unattended, and never in a garage or an enclosed space. If it starts when it should not, or will not shut down, isolate it rather than living with it — and on a vehicle people sleep in, treat that as urgent rather than inconvenient.
On a factory-fitted system the usual family rules apply and it is a dealer calibration job.
Likely causes
Start here
- Retrofitted equipment not configured · specialist
- Software updated on one module only · specialist
- Incorrect module configuration after replacement · specialist
Less common
- Module not coded to the vehicle · specialist
- Poor wiring splice in an aftermarket installation · moderate
- Software calibration out of date · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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