Lost Communication With ECM/PCM "B"
What it means
Another module, usually the main engine computer "A" or the transmission module, has stopped receiving messages from the second engine computer "B" over the CAN bus. Some vehicles have this second computer to control part of the engine or extra functions, and when it is lost the engine may run badly, with limited power, or not at all. The cause is usually computer power, ground, its connector or the bus wiring, and several U codes together point to a bus or battery problem.
Symptoms5
- Check engine light on with codes in several modules.
- Loss of power or engine in limp mode.
- Rough running or running on only some cylinders on some vehicles.
- Hard starting or no start in some cases.
- Scan tool cannot communicate with engine computer "B".
Possible causes6
- Weak battery or loose terminals.
- Blown power fuse or faulty main relay for computer "B".
- Poor computer ground connections.
- Corrosion or water in the computer connector.
- Open or shorted bus wiring between the two computers or to other modules.
- Internal failure of computer "B".
How to diagnose5
- Read codes from all modules; if many modules report lost communication, start with the battery and the bus, and if only the engine computer "B" is missing, focus on it.
- Try to communicate with the engine computer "B" directly with a scan tool; no response points to lost power, ground or wiring to it.
- Locate the engine computer "B" from the manufacturer information, which exists only on vehicles with two engine computers, such as some large engines with two cylinder banks or some diesels, and its role and location vary by make, then check its power fuse and measure power and ground at its connector.
- Inspect the module connector for water, corrosion or loose terminals, and check the bus wiring to it.
- Disconnect the battery and measure resistance between the CAN bus wires at the module connector, comparing with the manufacturer specification to find an open or short.
How to fix6
- Charge or replace the battery, and clean and tighten its terminals.
- Replace the blown fuse after finding why it blew.
- Clean the module connector and repair the ground or damaged bus wiring.
- Update the module software if the manufacturer has an update.
- Replace the engine computer "B" if proven faulty with good power and bus wiring, then program and code it to the vehicle.
- Clear codes in all modules and confirm the code does not return.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
The cost is low for a battery, fuse or ground fix, rises with wiring repairs, and is highest if computer "B" must be replaced and programmed to match the main computer.
Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026
Frequently asked questions
Why does my vehicle have two engine computers?
Some makers use a second computer to control part of the engine, such as one cylinder bank, or extra functions. Most vehicles do not have one.
How is this code different from U0100?
U0100 means lost communication with the main engine computer "A", while this code concerns the second computer "B". The test method is similar.
Can I drive with this code?
Only with caution for a short distance if the engine runs acceptably, because it may lose power or stall.
This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.
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