TCM Processor

Possible causes

• CommonLow or unstable supply voltage - a weak battery, a corroded earth, or a failing alternator makes a healthy module fail its own self-test
• CommonCorroded or water-ingressed module connector
• CommonWater reaching the module - many sit low in the engine bay or in a footwell
• CommonSoftware update interrupted, or programming never completed
• PossibleGenuine internal module failure
• PossibleDamage from a jump start with reversed leads, or from arc welding on the vehicle

What you might notice

• Check engine light on
• Limp mode or reduced power
• Engine will not start
• Several unrelated codes stored together
• Vehicle behaving unpredictably
• Warning lights coming and going

What to check, in order

1. Check battery voltage, the terminals and the earth straps before anything else. An unstable supply makes a perfectly good module fail its own self-test, and that is the most common reason this code appears on a module that turns out to be fine.
2. Look for water. Check the module's location and its connector for damp, corrosion and green residue - a blocked bulkhead or sunroof drain is a frequent cause.
3. Clear the code and see whether it returns. A single occurrence during a voltage dip is not the same as a module that fails every time it is powered up.
4. Ask what was done recently. An interrupted programming session, a jump start, or welding on the vehicle can all damage or confuse a module.
5. Treat module replacement as the last step, not the first. A replacement needs programming and coding to the vehicle, so a wrong guess is expensive twice over.