What this code means
P1659 may mean the PCM is detecting an electronic throttle control relay circuit condition that does not match the expected relay OFF behavior.
What the vehicle may do
- The vehicle may show throttle-related warning behavior.
- Throttle response may be reduced or limited in some cases.
- The condition can be intermittent if the code does not reset during verification.
Possible fault areas
- Possible ETCS control relay concern.
- Possible fuse or power feed concern.
- Possible open or short in relay-related wiring.
- Possible fuse and relay box or connector concern.
- Possible PCM software or PCM-related concern.
Diagnostic path
Opening context
On this Honda Civic 1.8 gas application, identified as 2015 Honda Civic Sedan L4-1.8L (R18Z1), P1659 points to an ETCS control relay OFF malfunction. In plain language, the PCM may not be seeing the electronic throttle control relay circuit behave the way it expects. The vehicle may have throttle-related warning behavior or reduced throttle response, depending on when the fault shows up. The broad fault areas can include the ETCS control relay, its fuse and power feeds, relay control wiring, the fuse and relay box connections, or a possible PCM software or PCM issue. Before diving in, start with the basic system checks and make sure any obvious connection or terminal problems are handled first.
Verify the fault before circuit testing
Start by turning the ignition ON, then clear the code with the HDS. After that, check for pending or confirmed codes and see whether P1659 comes right back. If it does, the failure is duplicated and you can move into the circuit checks. If it does not come back, treat it as intermittent for now. The system is OK at this time, so focus on poor connections or loose terminals at the ETCS control relay and the PCM. If captured operating data is available for this code, try to reproduce the same conditions instead of guessing.
Start with the fuse and relay path
With the ignition OFF, check fuse No. A22, rated at 15 A, in the under-hood fuse and relay box. If that fuse is OK, continue with the relay test path. If the fuse is not OK, skip ahead to the short-to-ground checks for the relay output and power feed circuits. For the normal fuse-OK path, remove the ETCS control relay from the under-dash fuse and relay box and test the relay. If the relay fails the test, replace the ETCS control relay. If the relay tests OK, keep going into the feed checks.
Fuse OK path: check the relay feeds
With the ETCS control relay still disconnected and the ignition ON, check the FI_MAIN_RLY_OUT circuit at ETCS control relay 4P socket No. 3 to body ground. You are looking for battery voltage. If battery voltage is present, move on to the +B_DBW feed. If battery voltage is not present, turn the ignition OFF, jump the SCS line with the HDS, disconnect PCM connector A, 51P, and under-dash fuse and relay box connector C, 38P. Then check continuity on the FI_MAIN_RLY_OUT circuit between PCM connector A No. 9, the ORN wire, and under-dash fuse and relay box connector C No. 9, also ORN. If continuity is present, that wire checks OK and the next supported repair is the under-dash fuse and relay box. If continuity is not present, repair the open in the FI_MAIN_RLY_OUT wire between PCM A9 and the ETCS control relay.
Continue the fuse OK path through DBW power and control wiring
Next, with the ignition ON and the relay disconnected, check the +B_DBW circuit at ETCS control relay 4P socket No. 1 to body ground for battery voltage. If battery voltage is missing, turn the ignition OFF, disconnect under-dash fuse and relay box connector D, 32P, and check the +B_DBW circuit at connector D No. 12, the YEL wire, to body ground. If battery voltage is present there, the +B_DBW wire checks OK and the next supported repair is the under-dash fuse and relay box. If battery voltage is not present, repair the open in the +B_DBW wire between the No. A22, 15 A fuse and the under-dash fuse and relay box. If battery voltage was present at the relay socket, turn the ignition OFF, jump the SCS line with the HDS, disconnect PCM connector C, 51P, and check continuity on the VBDBW and DBW_RLY_OUT circuit between PCM connector C No. 1, the WHT/GRN wire, and ETCS control relay 4P socket No. 2. If continuity is not present, repair the open between PCM C1 and the ETCS control relay. If continuity is present, disconnect PCM connector A, 51P, and check the DBW_RLY_CL- circuit between PCM connector A No. 50, the YEL wire, and ETCS control relay 4P socket No. 4. If that circuit is open, repair the open between PCM A50 and the ETCS control relay. If that circuit has continuity, the wiring checks OK; update the PCM if it does not have the latest software, or substitute a known-good PCM. If P1659 goes away after an update, troubleshooting is complete. If it goes away with the known-good PCM installed, replace the original PCM.
Fuse not OK path: look for shorts before condemning control parts
If fuse No. A22 was not OK, stay on the short-check path. There is one location detail to be careful with here: confirm you are working at the actual ETCS control relay cavity before testing, and do not guess if the vehicle layout does not match what you expect. With the ignition OFF, the ETCS control relay disconnected, the SCS line jumped with the HDS, and PCM connector C, 51P, disconnected, check continuity on the VBDBW circuit between PCM connector C No. 1, the WHT/GRN wire, and body ground. If there is continuity, repair the short in the VBDBW and DBW_RLY_OUT wire between PCM C1 and the ETCS control relay. If there is no continuity, that circuit checks OK. Next, with the relay still disconnected and PCM connector C still disconnected, check the +B_DBW circuit between ETCS control relay 4P socket No. 1 and body ground. If there is no continuity, update the PCM if needed, or substitute a known-good PCM. If P1659 goes away after an update, troubleshooting is complete. If it goes away with the known-good PCM installed, replace the original PCM. If there is continuity on the +B_DBW circuit, disconnect under-dash fuse and relay box connector D, 32P, and check continuity between connector D No. 12, the YEL wire, and body ground. If continuity remains, repair the short in the +B_DBW wire between the No. A22, 15 A fuse and the under-dash fuse and relay box. If continuity is gone, the +B_DBW wire checks OK and the supported repair is the under-dash fuse and relay box.
Verify the repair
After the supported repair, verify the repair and confirm P1659 stays gone. Keep the verification separate from the circuit testing: do not call it fixed just because one measurement looks good. The takeaway is simple: prove the code first, split the path by the fuse result, then isolate relay, power feed, control wiring, fuse and relay box, and PCM only in that order. For more diagnostic training, visit stepdiagnostics.com.
Final check
P1659 is best approached by verifying the code first, then following the fuse result into the correct relay, feed, wiring, fuse and relay box, or PCM check path.
For more guided automotive diagnostics, visit STEP Diagnostics.
