U0409: Invalid Data Received From Alternative Fuel Control Module
What to do
The vehicle will run on petrol and refuse to switch to gas. That fallback is deliberate — running a gas system on figures it cannot trust risks damaging the engine, so it uses the fuel it can be sure about.
Two innocent explanations to rule out before paying for anything.
Is the gas tank nearly empty? Low level and low pressure produce readings the system cannot make sense of, and filling it may simply resolve the whole thing.
Has the weather been extreme? Gas tank pressure changes a great deal with temperature, so a very hot or very cold spell can push a slightly drifted sensor outside what the system will accept. Worth re-testing in ordinary conditions.
If the gas system was fitted after you bought the vehicle, go to the installer rather than a main dealer. The module, its sensors and its wiring are their work, and they have the diagrams and equipment for it. Expect higher fuel costs meanwhile.
What this code means
The alternative fuel control module is broadcasting data that cannot be accepted as true. See U0400 for the family's framing and P2665 for the fuel shutoff side.
On a bi-fuel vehicle this module reports which fuel is in use, how much of the alternative fuel remains, its tank pressure and temperature, and whether the changeover is permitted — and the engine module needs all of it, because petrol and a gaseous fuel require different injection quantities and different ignition timing.
An implausible report therefore forces a specific and sensible response: the vehicle reverts to petrol and stays there. Running a gas system on fuelling figures that cannot be trusted risks running the engine lean, which on a gaseous fuel means high combustion temperatures and genuine engine damage. So the fallback is protection, and the loss of the cheaper fuel is the cost of it.
Tank pressure and temperature are the values most often at fault, and they have an honest explanation worth checking before anything is condemned. Gas tank pressure varies enormously with temperature — a tank in the sun reads very differently from the same tank on a frosty morning — and a sensor that has drifted, or a genuinely near-empty tank, produces readings the module cannot reconcile. A vehicle that developed this fault in extreme weather, or with the gas tank almost empty, is worth re-testing under ordinary conditions before parts are bought.
And the standing point from U0248 applies with full force here: much of this equipment is aftermarket. A conversion's module, its sensors and its wiring were installed by a specialist rather than the vehicle manufacturer. The installer is the right first port of call, and a main dealer may have neither the documentation nor the tooling.
The vehicle remains drivable throughout, on its other fuel.
Likely causes
Start here
- Faulty aftermarket alarm or immobiliser installation · moderate
- Corroded or loose connector · easy
- Damaged wiring harness · moderate
Less common
- Failed alternative fuel control module · specialist
- Faulty receiving module input · 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
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about U0409.