Network, generic

U0409 Invalid Data Received From Alternative Fuel Control Module

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

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.

Likely causes

Corroded connector or damaged wiring of the retrofit or factory gas system
Faulty gas rail pressure/temperature sensor delivering implausible values
Alternative fuel control module faulty or wrongly configured

Symptoms

  • Check engine light on
  • Engine stays on or switches back to petrol
  • Gas gauge or fuel mode indicator incorrect

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€100–900
Time1–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

alternative fuel control moduleengine control modulecan bus wiring

Related codes

U0108U0309U0406U0410

References: static.nhtsa.gov, static.nhtsa.gov, law.cornell.edu.