U0409Invalid Data Received From Alternative Fuel Control Module
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- U0409 is logged when the engine controller receives data from the alternative fuel control module that fail its validity checks. Such a module runs the gas side of bi-fuel CNG or LPG vehicles. In Ford's 2000 bi-fuel OBD description the AFCM passes the requested mode (ALT or GAS) and supply status between the gas metering unit and the PCM, which then switches the petrol injectors on or off; GM's 2013 CNG Silverado/Sierra sends rail pressure and temperature from its injector module to the ECM over serial data. No manufacturer documents U0409 publicly. Corrupt data can make the engine fall back to petrol or misjudge gas pressure, so CARB monitoring applies and a MIL is likely. U0108 means lost contact, U0309 a software mismatch.
- What you may notice
- Check engine light on
- Engine stays on or switches back to petrol
- Gas gauge or fuel mode indicator incorrect
- Likely causes
- LikelyCorroded connector or damaged wiring of the retrofit or factory gas system
- PossibleFaulty gas rail pressure/temperature sensor delivering implausible values
- PossibleAlternative fuel control module faulty or wrongly configured
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €100 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.