Sensor or wiring fault? How to tell FMI 3 from FMI 4

Many people see a sensor fault code and buy a new sensor straight away, only for the code to return after fitting. That's because the FMI number in generic J1939 codes describes the type of problem, and very often the problem is in the wiring, not the sensor.

What do FMI 3 and FMI 4 mean?

• FMI 3: the voltage from the sensor circuit is above normal. Usually an open signal wire, a wire shorted to a power wire, or a disconnected ground.
• FMI 4: the voltage is below normal. Usually the signal wire is shorted to the chassis, or there is water or moisture in the connector.

In both cases the ECU is telling you: the electrical signal from this sensor makes no sense. That's different from FMI 0 and FMI 1, which mean the actual reading is dangerously high or low (such as a real temperature or real oil pressure problem).

Checks before buying a sensor

1. Switch off the engine and unplug the sensor connector.
2. Inspect the connector: corrosion? water? bent or pushed-out pins? Clean it with electrical contact cleaner.
3. Follow the wiring from the sensor away from the engine: look for chafing or melting near the exhaust.
4. Reconnect the plug and make sure it clicks and locks. Clear the code and start the engine.
5. If the code returns, test the sensor with a multimeter against the manual's values, or have a technician check the full circuit.

Summary

FMI 3 and 4 tell you to check the electrics first. FMI 0 and 1 tell you the reading itself is wrong, and you need to deal with the actual problem. If the sensor turns out to be the cause, send us its number or a photo and we'll source it for you.