What does it mean?
P1674 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Engine Speed Ouput Circuit .
Symptoms
- MIL on while the engine itself runs normally
- Tachometer needle dead, stuck at zero, or bouncing and reading erratically
- Other functions that use the engine speed signal misbehave, such as remote start or some fan and A/C controls
- The tachometer may work intermittently and cut out over bumps
- Engine speed shown on a scan tool is correct while the gauge is not, which points straight at the output circuit
Likely causes
- Open or shorted engine speed output wire between the module and the cluster
- Corroded or backed-out pin at the module or cluster connector
- Failed tachometer circuit inside the instrument cluster
- An aftermarket device tapped into the tach signal: alarm, remote start, add-on gauge or shift light
- Blown instrument panel fuse
- Poor cluster ground causing an unstable signal reference
- Failed output driver inside the control module
How to fix it
- Compare engine speed on scan tool live data with the tachometer reading; a correct scan value confirms the fault is in the output circuit
- Check the instrument panel fuse and the cluster power and ground connections
- Unplug and inspect the cluster connector for corrosion, bent pins and weak terminal tension
- Test the engine speed wire for continuity from module to cluster and for shorts to ground and battery voltage
- Disconnect any aftermarket alarm, remote start or add-on gauge wired into the tach signal and retest
- View the signal with an oscilloscope if available, to confirm whether the module is producing a clean output
- Repair the wiring or the cluster circuit; consider the module output driver last