What this code means
P1658 may mean the PCM is seeing electronic throttle control relay power when the related subrelay should be off.
What the vehicle may do
- The vehicle may turn on the warning light.
- Throttle-control related symptoms can occur depending on when the fault is active.
- The vehicle may run normally at times if the fault is intermittent.
Possible fault areas
- Possible relay control circuit fault
- Possible wiring short or connector issue
- Possible PCM fault after related circuit checks are normal
Diagnostic path
Open on what P1658 is looking for
On this Civic, P1658 points to an electronic throttle control relay ON malfunction. In plain terms, the PCM may be seeing relay power when that subrelay should be off. The driver may only notice a warning light, or the car can have throttle-control related symptoms depending on when the fault shows up. The broad fault areas are possible relay control circuit problems, wiring shorts, connector issues, or, after the circuit checks pass, a possible PCM fault. Start with the basic system checks and follow a structured diagnostic approach before getting into the circuit testing.
Run the command test first
Put the vehicle in ON mode, then run the HDS ETCS, or TAC, test. This is the first split in the diagnosis. If that test does not pass, the failure has been duplicated, so stay on this diagnostic path instead of chasing unrelated possibilities. The logic behind the code is simple: if power supply voltage is present at the PCM even though the PGM-FI subrelay is being controlled off, the PCM treats that as abnormal and stores P1658.
Prepare before disconnecting the PCM
Before unplugging the PCM connector, use the HDS to jump the SCS line, then wait more than 1 minute. After that, prepare for the FI_SUB_RLY_OUT and FI_SUB_RLY_CL- circuit inspection with the PGM-FI subrelay disconnected and PCM connector A, the 50P connector, disconnected.
Check the relay control circuits
Now inspect the FI_SUB_RLY_OUT and FI_SUB_RLY_CL- circuits. Follow the open or shorted wire inspection method, and choose the inspection direction based on what the circuit is doing. At the disconnected PCM connector A 50P female terminals, check FI_SUB_RLY_OUT for a short to power with IG Mode ON. The normal result is not shorted. Then check FI_SUB_RLY_CL- for a short to ground with IG Mode OFF. The normal result there is also not shorted. If either inspection is abnormal, treat it as a circuit problem and repair the circuit fault before moving on.
Decision after the circuit checks
If the circuit inspections all show normal, the diagnostic path leaves the remaining PCM as the cause. At that point, pause and recheck the earlier diagnostic path before making a final call. Make sure the command test, the connector setup, and both short checks were done in the proper order, because guessing here can turn a wiring problem into an unnecessary module diagnosis.
Verify the repair
Keep verification separate from the circuit testing. After the fault area is corrected, verify the repair and confirm P1658 stays gone under the same kind of conditions that exposed the problem. The takeaway is: prove the relay command fault first, protect the PCM connector setup, check the two subrelay control circuits in order, and only consider the PCM after the wiring checks are normal. For more diagnostic training, visit stepdiagnostics.com.
Final check
Use the scan-tool command test first, then verify the relay control circuits before making a PCM call.
For more guided automotive diagnostics, visit STEP Diagnostics.
