Lost Communication With ECM/PCM "A"
What it means
Vehicle modules share data over a network such as the CAN bus. This code is set in another module, such as the transmission module (TCM), the ABS module or the instrument cluster, when messages from the engine computer (ECU) stop arriving. The cause is usually lost power or ground at the computer, or a network wiring fault.
Symptoms5
- No start or stalling in some cases.
- Several warning lights on the cluster at once.
- Temperature gauge or tachometer not working.
- Transmission in limp mode.
- Scan tool unable to communicate with the engine computer.
Possible causes6
- Weak battery or loose battery terminals.
- Blown engine computer power fuse or a faulty main relay.
- Poor ground connections for the computer.
- Broken, shorted or corroded network wiring or connectors.
- Another faulty module on the network disrupting communication.
- Internal engine computer failure.
How to diagnose6
- Check the battery and terminals and measure voltage while running.
- Try to connect directly to the engine computer with a scan tool; if it does not respond, check its power and ground first.
- Check the computer fuses and main relay, and measure power and ground at its connector.
- Measure the network line resistance at the diagnostic port and compare it with the manufacturer specification to find an open or short.
- If the network is disrupted, disconnect modules one at a time to find the one causing it.
- Inspect the computer connector for water or corrosion.
How to fix6
- Charge or replace the battery, and clean and tighten its terminals.
- Replace the faulty fuse or main relay.
- Repair damaged ground points or network wiring.
- Replace the module disrupting the network if proven faulty.
- Replace and program the engine computer if it is proven faulty with good power and network wiring.
- 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.
In the Saudi market: SAR 100–5,000 (≈ USD 27–1,330)
Our prices come from the Saudi market, converted at a fixed rate (SAR 3.75 per USD). Prices in your country may differ a lot, so we show the cost level.
The cost is low for a battery, fuse or ground fix, rises with extensive network wiring repairs, and reaches the upper end if the engine computer must be replaced and programmed.
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 the code appear in a module other than the engine computer?
Other modules are the ones that notice the engine computer's messages are missing, so they store this code. That is why it is usually read from the transmission, ABS or cluster module.
Does this code mean the engine computer is bad?
Not necessarily. Most cases come from a weak battery, fuse, ground or network wire, and the computer should only be condemned after those are checked.
Can I drive with this code?
If it appears intermittently and the engine runs, only with caution and for a short distance, because the engine may stall and systems such as the transmission and ABS can be affected.
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