P1673 — Fan output stage

Diagnostic algorithm

1. Connect a competent scan tool, read DTCs and freeze-frame data. Note engine coolant temperature and battery voltage when code set.
2. Perform quick visual inspection: fuses, relays, fan connectors, wiring for damage, corrosion or melting. Repair obvious issues and retest.
3. Using the scan tool, command the cooling fan ON and observe whether the module reports an output request and whether the fan responds.
4. While commanding the fan, check for battery voltage at the fan motor power terminal and for a good ground at the motor return. If no voltage at motor, trace back to relay/fuse and control module.
5. If voltage is present at the connector but fan does not run, disconnect fan and bench-test motor directly to battery (briefly, with proper safety) to confirm motor operation.
6. If motor runs when directly powered but not when commanded, inspect relay and control wiring. Verify relay function and replace if suspect. Check for PWM signal at module output when applicable.
7. If fan draws excessive current or stalls, replace fan motor assembly and retest.
8. If wiring, relay and motor are good but the control module does not provide correct output (no command or abnormal signal), suspect internal output stage failure in the module. Verify with wiring diagrams and consider module repair/replacement.
9. Clear codes, perform thermal test (run engine to normal operating temp with A/C on and verify fan operation) and confirm code does not reappear.
10. Replace only the failed component(s) identified by testing. After repair, clear codes and recheck system operation under load and temperature cycles.