GM Fuse Box

GM Fuse Box – Troubleshooting Guide

Important: Use this guide if you reinstalled your repaired GM Fuse Box and the problem is still happening.
Important: Please do not open/disassemble the fuse box—check fuses, relays, power/grounds, and wiring first.

• Step 1: Confirm installation
  • Verify the fuse box is fully seated/mounted and all connectors are locked
  • Inspect for bent/pushed pins, loose terminals, corrosion, or moisture at every connector
  • Make sure the harness isn’t pulled tight or rubbing (intermittent no-start is common from tension)
• Step 2: Battery and main power/ground checks
  • Confirm the battery is fully charged and passes a load test
  • Clean/tighten battery terminals and main chassis/engine grounds
  • Check the main power feed(s) into the fuse box for voltage drop during cranking
• Step 3: Re-check fuses and relays correctly
  • Confirm every related fuse has power on both sides (key OFF / key ON where applicable)
  • Swap known-good relays for the ECM/PCM power relay, fuel pump relay, and ignition relay (if applicable)
  • If a fuse blows again, stop and inspect wiring/components for a short before replacing more fuses
• Step 4: Address “No Start / Hard Start” complaints
  • Verify the starter cranks normally (if no crank, focus on starter circuit, ignition switch, neutral safety, and grounds)
  • If it cranks but won’t start, confirm fuel pressure and spark—don’t assume the fuse box is the cause
  • Check for intermittent power loss to the ECM/PCM (key on voltage at ECM/PCM feeds)
• Step 5: If code P0689 is present (ECM/PCM power relay sense)
  • Inspect the ECM/PCM power relay socket terminals for looseness/heat damage
  • Confirm the relay is being commanded ON and that the sense/feedback circuit is intact
  • Check for poor ignition feed, weak ground, or a failing ignition switch causing low relay voltage
• Step 6: Look for common vehicle-side causes that mimic a bad fuse box
  • Water intrusion in the under-hood electrical area
  • Loose battery cables, corroded grounds, or aftermarket wiring taps
  • Harness chafing near the radiator support/firewall where it can intermittently open/short
• Course of action
  • If the issue persists, contact UpFix with your VIN, full code list, and whether it’s “no crank” vs “crank/no start” so we can help narrow it down. Call or email for assistance.