P1674

What it means

P1674 on a GMC is a manufacturer-specific code meaning a fault in the tachometer control circuit. The engine control module drives or monitors the tachometer signal sent to the instrument cluster, and it has detected that the circuit is open, shorted or not behaving as expected. The exact wiring and which module is at fault vary by model and year.

Symptoms

• Tachometer reads zero, drops out or behaves erratically
• Tachometer needle stuck or jumping while the engine runs normally
• Check engine light or other warning lamps on
• Other instrument cluster gauges misbehaving at the same time
• Engine otherwise runs normally

Possible causes

• Damaged, chafed or corroded wiring in the tachometer signal circuit between the engine control module and the instrument clusterlikely
• Loose, corroded or spread terminals at the engine control module or cluster connectorslikely
• Fault in the instrument cluster tachometer driver or gaugepossible
• Poor power or earth supply to the engine control module or clusterpossible
• Short to voltage or earth in the circuit caused by an aftermarket accessory or previous repairpossible
• Internal fault in the engine control module output driverrare

How to diagnose

1. Read all stored codes and freeze frame data, noting whether other codes are present, then record and clear only after diagnosis
2. Check whether the tachometer works, and compare it against engine speed shown on a scan tool
3. Inspect the wiring and connectors between the engine control module and instrument cluster for chafing, water ingress, corrosion or poor terminal tension
4. Check fuses, power and earth supplies to the engine control module and instrument cluster
5. Using the manufacturer wiring diagram for the specific model, test the tachometer signal circuit for continuity, short to earth and short to voltage
6. Check for aftermarket electrical accessories or splices tied into the circuit
7. Observe the signal at the cluster with the engine running to confirm the module is sending a valid signal
8. If wiring and supplies are sound, test or substitute the cluster and then consider the engine control module, following model-specific procedures

How to fix

• Repair or replace damaged wiring in the tachometer circuit
• Clean or replace corroded or loose connector terminals
• Restore good power and earth connections to the modules
• Remove or correct faulty aftermarket wiring
• Repair or replace the instrument cluster if the tachometer driver is faulty
• Repair or replace the engine control module only after the circuit and cluster are proven good, with any required programming

Can I keep driving?

This code for other makes

• P1674BuickTachometer Control Circuit
• P1674CadillacTachometer Control Circuit
• P1674ChevroletTachometer Control Circuit
• P1674GMTachometer Control Circuit
• P1674OldsmobileTachometer Control Circuit
• P1674SaturnTachometer Control Circuit

Nearby codes

• P1673GMCEngine Hot Lamp Control Circuit
• P1675GMCEVAP Vent Solenoid Control Circuit