What this code means
B1275 may indicate a headlight switch position circuit malfunction where the MICU may not be seeing the expected combination light switch input.
What the vehicle may do
- Exterior lights may not match the selected switch position.
- Scan data may show a headlight switch input stuck on or stuck off.
- The issue may be intermittent if the switch data is correct during testing.
Possible fault areas
- Combination light switch
- Switch signal wiring
- Switch ground circuit
- Loose or poor connector connections
- MICU in the under-dash fuse and relay box
Diagnostic path
Opening context
On this Accord, B1275 is a headlight switch position circuit malfunction handled through the MICU. In plain terms, the module may not be seeing the combination light switch position match what the driver selected. The vehicle may have exterior light operation that does not line up with the switch position, or the scan data may show one headlight switch input stuck on or stuck off. The possible fault areas can include the combination light switch, its ground and signal wiring, connector fit or terminal condition, and the MICU inside the under-dash fuse and relay box. We are going to diagnose it by watching the switch inputs on the HDS, then using the wiring checks only after the scan data tells us which direction to go.
Confirm the switch data first
Start by verifying the problem in the data list. Watch Headlight Switch OFF, PARKING, AUTO, and HEADLIGHT while moving the combination light switch through each position. With the switch OFF, the OFF input should read ON, and PARKING, AUTO, and HEADLIGHT should read OFF. In POSITION, or PARKING, OFF should read OFF, PARKING should read ON, AUTO should read OFF, and HEADLIGHT should read OFF. In AUTO, OFF and PARKING should read OFF, AUTO should read ON, and HEADLIGHT should read OFF. With the switch ON, or HEADLIGHT, OFF should read OFF, PARKING should read ON, AUTO should read OFF, and HEADLIGHT should read ON. If all of that data is correct, treat it as an intermittent at this time. The system is OK right now, so concentrate on loose or poor connections. If any value is wrong, continue into the circuit diagnosis.
Decide whether the input is stuck on or stuck off
Next, keep watching the same four HDS parameters: Headlight Switch OFF, PARKING, AUTO, and HEADLIGHT. The key question here is whether the problem value is OFF instead of ON. If the data list value is OFF when it should not be, go to the stuck-off side of the diagnosis. If it is not OFF, and the input is staying ON, go to the stuck-on side first.
If the switch input is stuck on
For a value that is stuck ON, turn the ignition switch to LOCK (0), or select OFF mode with the engine start stop button. Disconnect the combination light switch 12P connector, then look at the same HDS switch parameters again. If the data changes from ON to OFF with the switch disconnected, the combination light switch is the failed area, and the directed repair is to replace the combination light switch. If the data does not change from ON to OFF, keep going and check the wiring for a short to ground.
Check the stuck-on circuits for a short to ground
With the ignition still at LOCK (0), or in OFF mode, leave the combination light switch 12P connector disconnected and disconnect under-dash fuse and relay box connector N (20P). Now check continuity to body ground on the three switch signal lines one at a time. For H/L OFF SW, check connector N (20P) No. 18, PNK, to body ground. For H/L ON SW, check connector N (20P) No. 19, YEL, to body ground. For PARKING LT SW, check connector N (20P) No. 20, GRY, to body ground. If there is continuity, repair the short to ground in that wire. If there is no continuity, those three wires are OK, and the directed result is a faulty MICU; replace the under-dash fuse and relay box.
If the switch input is stuck off
For a value that is stuck OFF, turn the ignition switch to LOCK (0), or select OFF mode with the engine start stop button. Disconnect the combination light switch 12P connector. Then use a jumper wire to ground each listed switch input at the combination light switch connector, and watch the HDS data. Ground No. 1, PNK, to body ground; ground No. 7, YEL, to body ground; and ground No. 8, GRY, to body ground. If the data changes from OFF to ON, the combination light switch is the failed area, and the directed repair is to replace the combination light switch. If the data does not change from OFF to ON, move on to the open-circuit checks.
Check the stuck-off circuits for an open or high resistance
For the open-wire check, keep the ignition at LOCK (0), or in OFF mode. Disconnect under-dash fuse and relay box connector N (20P), and keep the combination light switch 12P connector disconnected. Check continuity end to end on the affected switch circuits and the light switch ground. For H/L OFF SW, check connector N (20P) No. 18, PNK, to combination light switch 12P connector No. 1, PNK. For H/L ON SW, check connector N (20P) No. 19, YEL, to combination light switch 12P connector No. 7, YEL. For PARKING LT SW, check connector N (20P) No. 20, GRY, to combination light switch 12P connector No. 8, GRY. For COMBI SW GND (LT), check connector N (20P) No. 1, BLK, to combination light switch 12P connector No. 6, BLK. If continuity is present, those circuits are OK, and the directed result is a faulty MICU; replace the under-dash fuse and relay box. If continuity is not present, repair the open or high resistance in the wire.
Verify and close
After the repair, keep verification separate from the circuit testing. Go back to the original switch-position data check and make sure the HDS values now follow each switch position correctly. The takeaway on B1275 is simple: prove the scan data first, decide whether the input is stuck on or stuck off, then test only the circuit path that matches that failure. For more diagnostic training, visit stepdiagnostics.com.
Final check
B1275 is best approached by proving the switch-position data first, then following the stuck-on or stuck-off circuit path.
For more guided automotive diagnostics, visit STEP Diagnostics.
