What this code means
This code may mean the climate control unit is not receiving the expected sunlight sensor circuit input.
What the vehicle may do
- Automatic climate control may not compensate for sun load correctly.
- Cabin temperature response can seem inconsistent when sunlight changes.
- The system may store the fault even when the symptom is subtle.
Possible fault areas
- Possible sunlight sensor fault.
- Possible open or poor connection in the sensor circuit wiring.
- Possible connector or terminal fit issue at the sensor or climate control unit.
- Possible climate control unit fault after the sensor and wiring checks pass.
Diagnostic path
Opening context
On this 2010 to 2015 Honda Civic, B1229 is for an open in the sunlight sensor circuit. In plain terms, the climate control unit may not be seeing the sunlight sensor signal correctly. The customer may notice the automatic climate control acting a little off, especially when cabin temperature correction depends on sun load. The possible fault areas are the sunlight sensor itself, the TSUN signal wire, the SENSOR_COM wire, connector fit or terminal condition, and only after the circuit checks pass, the climate control unit. We are going to work this in order and let each result decide the next move.
Confirm whether the fault is current
Start by clearing the DTC with the HDS. Turn the ignition switch to LOCK (0), then back to ON (II). Run the self-diagnostic with the HDS or with the climate control unit, then check for codes again. If the B1229 fault comes back, keep going with the sunlight sensor check. If it does not come back, treat it as an intermittent failure and inspect for loose wires or poor connections in the sunlight sensor circuit before going any deeper.
Check the sunlight sensor
Next, test the sunlight sensor. If the sensor checks OK, move on to the wiring checks. If the sunlight sensor does not check OK, replace the sunlight sensor. Do not condemn the control unit before this sensor check is handled.
Check the TSUN wire for an open
Now check the TSUN circuit for an open. Turn the ignition switch to LOCK (0). Disconnect the climate control unit 32P connector. At the sunlight sensor, disconnect the 7P connector if the vehicle has automatic lighting, or the 2P connector if it does not. Check continuity on the TSUN wire between the climate control unit 32P connector terminal No. 21, PUR, and the sunlight sensor terminal No. 2, PUR. That terminal No. 2 applies whether the sensor connector is the 7P version or the 2P version. If there is continuity, the TSUN wire is OK and you move to the SENSOR_COM check. If there is no continuity, repair the open in the wire between the climate control unit and the sunlight sensor.
Check the SENSOR_COM wire for an open
With the connectors still disconnected and the ignition switch at LOCK (0), check the SENSOR_COM circuit. On vehicles with automatic lighting, check continuity between the climate control unit 32P connector terminal No. 20, RED, and the sunlight sensor 7P connector terminal No. 3, RED. On vehicles without automatic lighting, check continuity between the climate control unit 32P connector terminal No. 20, RED, and the sunlight sensor 2P connector terminal No. 1, RED. If there is no continuity, repair the open in the wire between the climate control unit and the sunlight sensor. If there is continuity, the SENSOR_COM wire is OK. At that point, with the sunlight sensor already checked OK and both circuit paths showing continuity, replace the climate control unit.
Wrap-up and verification
After the sensor, wiring, or control unit issue has been addressed, use the same clear-and-self-diagnostic routine to verify the repair and confirm the B1229 fault stays gone. The takeaway is simple: verify the code first, check the sunlight sensor, prove the TSUN and SENSOR_COM wires, and only then make the control unit call. For more diagnostic training, visit stepdiagnostics.com.
Final check
Follow the circuit path in order: confirm the fault, test the sensor, check both wires for continuity, then make the final call based on the test results.
For more guided automotive diagnostics, visit STEP Diagnostics.
