GM ECU/ECM

GM ECU/ECM Post-Repair Guide

Important: Use this guide if you reinstalled your repaired GM ECU/ECM and the problem is still happening.
Important: Please do not open/disassemble the module. This can damage the unit and may void warranty coverage.

Step 1: Battery & resets

• Fully charge the battery (low voltage can cause false ECU faults)
• Clear codes, then recheck after a short drive
• If you installed a different ECU, confirm it matches and is programmed for the vehicle
  Step 2: Check power, grounds, and fuses
• Verify all ECU/IGN/INJ/PCM fuses in underhood + interior panels
• Inspect ECU grounds at the engine block and frame (clean/tight, no rust/paint)
• Check for loose battery terminals and main power feed connections
  Step 3: Connector inspection (most common comeback cause)
• Key OFF, unplug ECU connectors
• Check for bent pins, pushed-out pins, moisture, corrosion, or damaged seals
• Confirm connectors are fully seated and lever locks are fully latched
  Step 4: If you have misfires/rough idle
• Check ignition coils, plugs, and plug wires (if equipped)
• Verify injector connectors are fully seated; inspect injector harness for rub-through
• Fix any vacuum leaks (intake boot, PCV, brake booster line, intake gaskets)
  Step 5: If you have timing/engine power loss complaints
• Inspect crankshaft and camshaft sensor connectors and wiring
• Check for damaged harness near exhaust manifolds and at the rear of the engine
• Confirm the throttle body and MAF connectors are secure and clean
  Step 6: If you have “no start” or intermittent stall
• Verify fuel pressure meets spec (weak pump/relay can mimic ECU issues)
• Check the main relay / ECM relay operation and relay socket pin fit
• Inspect grounds and the harness near the fuse box for heat damage
  Step 7: If codes return immediately after clearing
• A local shop should verify power/ground integrity under load and confirm sensor signals
• Multiple unrelated codes often point to wiring, grounds, or power supply issues
  If the issue started right after wiring/sensor/fuse/relay work, re-check the exact areas that were serviced first. If problems continue, contact UpFix with your VIN, full code list, and what was found in the fuse/ground/connector checks.