What this code means
P1659 may mean the PCM is seeing a possible mismatch between commanded relay operation and the power supply input for the electronic throttle control system.
What the vehicle may do
- The warning light may come on.
- Throttle response may feel limited or abnormal.
- The vehicle can enter a reduced-power or fail-safe type strategy.
Possible fault areas
- Possible power feed issue.
- Possible PGM-FI subrelay control or output circuit issue.
- Possible connector or terminal condition issue.
- Possible PCM-side issue after circuit checks are proven normal.
Diagnostic path
Opening context
On this 2018 to 2025 Honda Accord 2.0 Hybrid, P1659 is an electronic throttle control system relay off malfunction. In plain language, the PCM may be commanding the PGM-FI subrelay on, but it is not seeing the power supply input it expects for the throttle control side. The vehicle may turn on the warning light, may limit throttle response, or may feel like it is in a reduced-power strategy. Broadly, think possible power feed, relay control wiring, relay output wiring, connector condition, or a PCM-side issue. The diagnostic logic is simple: first confirm the code condition, then prove the power feed, prove the relay control and output circuits are not open, prove the output is not shorted to ground, and only then judge the PCM.
Confirm the setting logic
Start by turning the vehicle to the ON mode. The setting logic for P1659 is that there is no power supply voltage input to the PCM even though the PGM-FI subrelay is being controlled on. So do not start by assuming a module problem. Treat this as a controlled relay power-supply diagnosis and work the circuit in order.
Check the +B_DBW power feed first
Begin the +B_DBW circuit inspection with the PGM-FI subrelay disconnected and the IG mode off. The two +B_DBW power checks at the relay socket are both expected to show 12 V battery voltage. If either one is abnormal, the fault is in the circuit, and the next move is the additional power supply inspection to locate where that feed is being lost. Because the exact relay-socket terminal identification matters here, positively identify the terminals before probing and do not guess at cavity locations.
Check the relay control and output for opens
If the +B_DBW power checks are normal, move to the open-circuit checks for FI_SUB_RLY_CL- and FI_SUB_RLY_OUT. Before disconnecting the PCM connector, jump the SCS line with the HDS and wait more than 1 minute. Then continue with the PGM-FI subrelay disconnected, PCM connector A 72P disconnected, and IG mode off. The FI_SUB_RLY_CL- open-wire check should be less than 1 Ω, and the FI_SUB_RLY_OUT open-wire check should also be less than 1 Ω. If either check is abnormal, the problem is in that circuit. Again, confirm the correct PCM and relay terminals before measuring.
Check FI_SUB_RLY_OUT for a short to ground
Next is the short-to-ground check on FI_SUB_RLY_OUT. Before disconnecting the PCM connector for this part of the test, jump the SCS line with the HDS and wait more than 1 minute. Set the circuit up with the PGM-FI subrelay disconnected, PCM connector A 72P disconnected, the applicable EVAP purge valve connectors disconnected for the market you are working on, the EVAP leak check module disconnected where used, and the A/F sensor Sensor 1 connector disconnected. Use the normal shorted-wire inspection method and inspect according to the circuit condition. With IG mode off, FI_SUB_RLY_OUT should measure more than 1 MΩ to ground. If it does not, the fault is in the circuit.
PCM judgment and closeout
If the +B_DBW power supply checks are normal, the relay control and output circuits are not open, and FI_SUB_RLY_OUT is not shorted to ground, the remaining determination in this path is a problem with the PCM. Keep that as the end of the diagnostic path, not the starting point. After correcting the fault indicated by the test results, verify the repair and confirm the code stays gone. The takeaway is to prove the relay power and wiring first, then let the circuit results decide whether the PCM is left as the issue. For more diagnostic training, visit stepdiagnostics.com.
Final check
P1659 is often best approached as a relay power and wiring diagnosis before considering the PCM.
For more guided automotive diagnostics, visit STEP Diagnostics.
