GM Duramax FICM Repair

GM Duramax 6.6L FICM (LB7/LLY) – Troubleshooting Guide

Important: Use this guide if you reinstalled your repaired GM Duramax 6.6L Fuel Injection Control Module (FICM) and the problem is still happening.
Important: Please do not open/disassemble the module—check vehicle power, grounds, wiring, fuses, and injector circuits instead.

• Step 1: Confirm install basics
  • Verify all FICM connectors are fully seated and locked
  • Inspect for bent/pushed pins, corrosion, or loose terminals at the FICM and harness
  • Ensure the harness is not stretched, pinched, or rubbing on brackets
• Step 2: Battery voltage, grounds, and charging
  • Confirm batteries are fully charged and pass a load test
  • Clean/tighten battery terminals and engine/chassis grounds
  • Weak voltage or poor grounds can cause injector/coil-style circuit codes and misfires
• Step 3: Verify power supply and fuses
  • Check all engine control and injector-related fuses/relays feeding the FICM/ECM circuits
  • If a fuse blows again, stop and inspect wiring for a short before replacing more fuses
  • Inspect the under-hood fuse block for heat damage or loose terminals
• Step 4: If you have misfires / rough run
  • Use scan data to identify which cylinder(s) are affected
  • Inspect injector connectors and harness sections for rubbed insulation, oil saturation, or prior repairs
  • If the issue is isolated to 1–2 cylinders, it’s more often injector/harness than the module
• Step 5: If you have injector circuit codes (example: P0201, etc.)
  • These commonly point to wiring/connector or injector coil issues on that cylinder
  • Check the injector connector for spread terminals or poor pin fit
  • Have a shop perform circuit integrity checks from the module/engine harness to the injector
• Step 6: If you have “no start” or long crank
  • Don’t assume it’s the FICM—verify fuel supply and pressure first
  • Confirm the engine has RPM signal during cranking (no RPM can stop injector command)
  • If fuel supply is OK, scan for security/communication faults and confirm ECM is powering up normally
• Step 7: If the issue is intermittent
  • Wiggle-test the FICM harness/connectors while monitoring symptoms
  • Inspect harness routing near hot or sharp areas (brackets, valve cover edges, firewall)
  • Re-check any areas that were recently serviced (battery work, injector work, harness repair)
• Course of action
  • If the problem persists, contact UpFix with your VIN, full code list, and exact symptoms (no-start vs misfire) so we can help narrow it down. Call or email for assistance.