Ford Mustang / Freestar GEM / SJB Troubleshooting Guide

Important: Use this guide if you reinstalled your repaired (Ford Mustang/Freestar GEM/SJB) and the problem is still happening. Please do not open/disassemble the module. Opening it can cause damage and may void warranty support.

Step 1: Confirm the exact symptom

• • No communication with GEM/SJB (U-codes), or intermittent communication
  • Interior/courtesy lights not working (or stuck on)
  • Door locks/keyless not working (or random lock/unlock)
  • Intermittent power/reset issues (functions cut out, then return)
  • Note if it affects everything or only one area (locks only, lights only, etc.)

Step 2: Verify battery voltage and charging health

• • Low voltage can cause resets, false U-codes, and erratic body functions
  • If the battery was recently dead/jump-started, fully charge it and re-test
  • Confirm battery terminals are tight/clean and grounds are secure

Step 3: Check fuses related to body electronics

• • Check all fuses that feed GEM/SJB, interior lighting, locks/keyless, and CAN/body circuits
  • If a fuse is blown, replace it once and re-test
  • If it blows again, stop—inspect wiring and the affected circuit for a short

Step 4: Inspect connectors and wiring at the module

• • Unplug/replug connectors and confirm they fully seat and lock
  • Look for corrosion/moisture, bent pins, backed-out pins, or overheated terminals
  • If the issue started after wiring/fuse/relay work, recheck what was touched first

Step 5: If the issue is “No Communication” / U-codes

• • Scan the vehicle and note whether multiple modules show U-codes
  • If multiple modules have U-codes, focus on CAN wiring/power/ground/shared fuses first
  • If only the GEM/SJB won’t communicate, focus on its power/ground feeds and connector pin fitment

Step 6: If door locks/keyless issues remain

• • Test lock/unlock from multiple inputs (remote, key, interior switch)
  • If only one door misbehaves, inspect that door’s actuator and the door-jamb wiring boot first
  • Door-ajar switches and broken door-jamb wires are common causes of “random” lock behavior

Step 7: If interior/courtesy light issues remain

• • Check for water intrusion (wet carpet/kick panel/cowl area) and fix the leak first
  • Verify the dimmer switch operation and that no bulb/circuit is shorting
  • If lights stay on, confirm door-ajar inputs are reporting correctly

If these steps don’t resolve the issue, please contact UpFix with your scan results (codes) and what checks you completed so we can help narrow it down faster.