What this code means
P1659 may mean the PCM is not seeing the expected relay-fed power supply for the electronic throttle control side when the relay is being commanded on.
What the vehicle may do
- The vehicle may show throttle-control warnings.
- The vehicle may have reduced power or drivability issues.
- The condition can be intermittent if the relay feed or wiring connection is unstable.
Possible fault areas
- Possible PGM-FI subrelay power feed or relay-output circuit concern.
- Possible open or shorted wiring in the relay control or output circuits.
- Possible connector or terminal issue in the affected circuit path.
- Possible PCM concern after the circuit checks are normal.
Diagnostic path
Open on P1659
For this 2016 through 2024 Honda Civic with the 2.0 gas engine, P1659 points us toward the electronic throttle control power supply side, around the PGM-FI subrelay and the PCM. In plain terms, the PCM may be commanding that relay on, but not seeing the power supply voltage it expects. Depending on the failure, the car may have throttle-control warnings, reduced power, a no-start, or an intermittent drivability issue. The broad fault areas are possible relay feed problems, open or shorted wiring, connector issues, or, after the circuit checks pass, a possible PCM problem.
Start with confirmation and the +B DBW feed
Start by turning the vehicle to ON mode. Then disconnect the PGM-FI subrelay and check the +B_DBW circuit at the disconnected PGM-FI subrelay 4P socket with IG Mode OFF. Normal is 12 V battery voltage. If that feed is not normal, stay in the power supply side of the diagnosis and locate the circuit fault before moving on.
Check the relay control and output circuits for opens
Before disconnecting the PCM connector, jump the SCS line with the HDS and wait more than 1 minute. Then disconnect the PGM-FI subrelay and PCM connector A, the 50P connector. With IG Mode OFF, check the FI_SUB_RLY_CL- and FI_SUB_RLY_OUT circuits between the disconnected PGM-FI subrelay 4P socket and the disconnected PCM connector A terminals. Each open-wire check should show less than 1 Ω. If either one is abnormal, the problem is in that circuit.
Check FI_SUB_RLY_OUT for a short to ground
For the shorted-wire check, again protect the PCM connector step first: jump the SCS line with the HDS and wait more than 1 minute before disconnecting the PCM connector. Then disconnect the PGM-FI subrelay, PCM connector A, the A/C condenser fan relay, the fan control relay, the A/F sensor Sensor 1 6P connector, and, on USA and Canada models, the EVAP canister vent shut valve 2P connector. Use the correct open-or-shorted wire inspection method for the circuit, and check FI_SUB_RLY_OUT at the disconnected PGM-FI subrelay 4P socket with IG Mode OFF. Normal is not shorted. If it is shorted, the fault is in the circuit. The terminal locations matter here, so if you cannot positively identify the terminals, pause, recheck the earlier diagnostic path, and do not guess.
When the circuit checks are normal
If the +B_DBW feed check is normal, the open-wire checks are normal, and the FI_SUB_RLY_OUT short-to-ground check is normal, the remaining determination is that there is a problem with the PCM. Do not jump to that conclusion early; this code is built around proving the relay power feed, the control and output wiring, and the short-to-ground check first. Takeaway: P1659 is diagnosed by proving whether the PCM’s expected relay-fed power path is present, open, shorted, or only left with the PCM after the circuit tests pass. For more diagnostic training, visit stepdiagnostics.com.
Final check
Work the circuit in order: confirm the feed, prove the open-wire checks, prove the short-to-ground check, and only then consider the PCM as the remaining cause.
For more guided automotive diagnostics, visit STEP Diagnostics.
