CAN network

U0402 — Invalid Data Received from Engine Control Module (ECM/PCM)

Severity: medium Drive with care, to a shop

What U0402 means

U0402 is set when another module, such as the instrument cluster, transmission or ABS controller, receives messages from the engine control module that fail plausibility checks. The ECM is still communicating on the network, but the values it sends don't make sense. Under normal conditions these modules constantly share this data over the CAN bus and rely on it to operate.

Symptoms

  • Erratic or wrong gauge readings
  • Limp mode with reduced power
  • Check Engine light on
  • Several other invalid-data U-codes at once
  • Harsh or odd shifting
  • Unstable idle

Possible causes

  1. Failing ECM/PCM
  2. Faulty sensor inputs corrupting the data the ECM broadcasts
  3. Software bug in the ECM
  4. Low system voltage corrupting network messages
  5. Poor or corroded ECM ground
  6. Damaged wiring or connectors at the ECM

How to diagnose

  1. Scan all modules and record freeze frame data; note which code was set first
  2. Test battery voltage and charging voltage with the engine running
  3. Inspect battery terminals and engine and body ground points
  4. Review ECM live data for implausible sensor values
  5. Inspect ECM connectors and harness for moisture, corrosion or damage
  6. Check the CAN bus: resistance between CAN-H and CAN-L and wire continuity
  7. Look for available ECM software updates
  8. Clear codes and see whether U0402 returns

Common fixes

  • Charge or replace the battery and repair any charging system fault
  • Clean or repair grounds and connectors
  • Replace the faulty sensor corrupting the data
  • Reflash or update the ECM software
  • Repair the damaged wiring harness
  • Repair or replace the ECM only after everything else is ruled out

Questions and answers

What does U0402 mean?

It means a module on the network received data from the ECM/PCM that failed validity checks. Communication exists, but the content of the messages is wrong.

Do I need to replace the ECM for U0402?

Not right away. Replacement is a last resort. First check battery voltage, grounds, wiring, sensors and whether a software update is available.

Can a weak battery cause U0402?

Yes. Low voltage makes modules behave erratically and can corrupt network messages, which sets invalid-data U-codes.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.