Trouble code
U0100
Lost Communication With ECM/PCM A
What U0100 means
A module has lost communication with the engine computer over the vehicle's data network.
Symptoms you may notice
- Multiple warning lights
- No start
- Gauges dead or erratic
- Several unrelated codes
Common causes, cheapest first
- Low battery voltage
- Corroded or loose grounds
- Damaged CAN bus wiring
- Failed module
- Water intrusion into a connector
Before you buy anythingTest the battery and clean every ground strap before chasing modules. Low voltage causes network communication faults far more often than a failed computer does.
What to do next
- Confirm the code with your own scan tool, or have it read free at a chain parts store.
- Decode your VIN so you know your exact engine and build date.
- Work down the causes above from cheapest to most expensive.
- Search NHTSA for a recall or service bulletin covering this symptom on your vehicle.
- Read the batteries buying guide before ordering.
U0100 is a generic OBD-II code, standardized across every make and model sold in the US from 1996 onward. The meaning is the same on any vehicle; the specific part and its location are not.
Parts
Parts commonly needed for U0100
These go to Amazon. We earn a commission if you buy, at no extra cost to you — see our affiliate disclosure. Which one you actually need depends on diagnosis — read the note above first.
Confirm fitment against your VIN on the listing page before ordering. A part that matches your year and model can still be wrong for your build date or engine code.