What this code means
B1275 may mean the MICU is seeing a headlight switch position signal that does not agree with the expected switch state.
What the vehicle may do
- Exterior lights may not respond as expected to the combination light switch.
- The concern can be intermittent and may show normal operation during testing.
- A lighting-related body electrical code may be stored.
Possible fault areas
- Possible combination light switch concern.
- Possible short to ground in a switch signal circuit.
- Possible open or high resistance in a switch signal or switch ground circuit.
- Possible MICU or under-dash fuse/relay box involvement.
Diagnostic path
Opening context
On this 2010 through 2015 Honda Civic 1.8, B1275 is a headlight switch position circuit fault being seen by the MICU in the body electrical lighting system. In plain language, the controller may be seeing a combination light switch signal that does not match the position the switch is actually in. The exterior lights may not respond the way the driver expects, or the problem may be intermittent. Broadly, this can come from the combination light switch, a short to ground, an open or high-resistance circuit, or possible MICU and under-dash fuse/relay box involvement. The key is to use the scan-tool data first, then prove whether the circuit is stuck on, unable to pull on, or intact back to the MICU.
Verify the switch data first
Start with the lighting data list. Move the combination light switch through OFF, PARKING, AUTO, and ON (HEADLIGHT), and compare each displayed headlight switch value to the expected pattern. In OFF, Headlight Switch (OFF) should be ON, and Headlight Switch (PARKING), Headlight Switch (AUTO), and Headlight Switch (HEADLIGHT) should be OFF. In PARKING, Headlight Switch (PARKING) should be ON, and the other three should be OFF. In AUTO, all four listed headlight switch values should be OFF. In ON (HEADLIGHT), Headlight Switch (PARKING) and Headlight Switch (HEADLIGHT) should be ON, while Headlight Switch (OFF) and Headlight Switch (AUTO) should be OFF. If all of that matches, treat it as an intermittent at this time and check for loose or poor connections. If it does not match, keep going.
Decide if the data value is stuck on
Next, stay in the data list and check the combination light switch position value that is not behaving correctly. The question here is simple: is the data list value OFF, not ON? If it is OFF when you need to prove the circuit can turn ON, move to the jumper simulation test. If the value is ON when it should not be, go after a possible stuck-on signal next.
If the signal is stuck on, unplug the switch
For a value that is stuck ON, turn the ignition switch to LOCK (0), disconnect the Combination light switch 12P connector, and check that same data list value again. If the value changes from ON to OFF with the switch unplugged, the diagnostic path points to the combination light switch, and the repair is to replace the combination light switch. If the value does not change from ON to OFF, leave the switch disconnected and move to the short-to-ground check.
Check the stuck-on circuits for a short to ground
With the ignition switch at LOCK (0), the Combination light switch 12P connector disconnected, and Under-dash fuse/relay box connector T (20P) disconnected, check continuity to body ground on the switch signal lines individually. Check H/L_OFF_SW at Under-dash fuse/relay box connector T (20P) No. 1 (ORN) to body ground. Check H/L_ON_SW at connector T (20P) No. 2 (PNK) to body ground. Check PARKING_LT_SW at connector T (20P) No. 3 (GRY) to body ground. If there is continuity, repair the short to ground in that wire. If there is no continuity, the diagnostic path points to a faulty MICU, and the repair is to replace the under-dash fuse/relay box.
If the value is off, simulate the switch input
If the data list value is OFF and you need to see whether the MICU can recognize the switch input, use a jumper wire from the Combination light switch 12P connector to body ground. Connect No. 1 (ORN) to body ground for the H/L_OFF_SW line, No. 7 (PNK) to body ground for the H/L_ON_SW line, and No. 8 (GRY) to body ground for the PARKING_LT_SW line. Watch the data list while you do this. If the value changes from OFF to ON, the diagnostic path points to the combination light switch, and the repair is to replace the combination light switch. If it does not change from OFF to ON, continue to the open-circuit check.
Check for an open or high resistance
For the open-circuit side of the diagnosis, turn the ignition switch to LOCK (0), disconnect Under-dash fuse/relay box connector T (20P), and check continuity between the under-dash fuse/relay box and the combination light switch circuits. Check H/L_OFF_SW from connector T (20P) No. 1 (ORN) to Combination light switch 12P connector No. 1 (ORN). Check H/L_ON_SW from connector T (20P) No. 2 (PNK) to Combination light switch 12P connector No. 7 (PNK). Check PARKING_LT_SW from connector T (20P) No. 3 (GRY) to Combination light switch 12P connector No. 8 (GRY). Also check COMBI_SW_GND (LT) from connector T (20P) No. 5 (BLK) to Combination light switch 12P connector No. 6 (BLK). If there is continuity through the checked circuits, the diagnostic path points to a faulty MICU, and the repair is to replace the under-dash fuse/relay box. If there is not continuity, repair the open or high resistance in the affected wire.
Verification and takeaway
Keep verification separate from the fault isolation. After the indicated repair, verify the repair by checking that the headlight switch data now follows the switch positions correctly and confirm B1275 stays gone. The takeaway is not to guess at the switch or the MICU; let the data list, the ground simulation, and the continuity checks tell you which side of the circuit has failed. For more diagnostic training, visit stepdiagnostics.com.
Final check
Use scan-tool switch data first, then circuit checks to separate a switch problem from wiring or MICU involvement.
For more guided automotive diagnostics, visit STEP Diagnostics.
