What this code means
B1225 may indicate the climate control unit is seeing an abnormal in-car temperature sensor signal, often consistent with a possible open in that sensor circuit.
What the vehicle may do
- Automatic climate control may respond incorrectly to cabin temperature.
- Cabin temperature correction can seem inaccurate or inconsistent.
Possible fault areas
- Possible in-car temperature sensor circuit wiring issue.
- Possible connector or terminal condition issue.
- Possible SENSOR_COM circuit issue.
- Possible climate control unit issue after the circuit checks normal.
Diagnostic path
Open the diagnosis
On this Civic, B1225 is a climate control code tied to the in-car temperature sensor circuit. In plain language, the climate control unit may be seeing that sensor input go higher than expected, and that can happen when the in-car temperature sensor circuit is open. The customer may notice automatic climate control behavior that does not match cabin conditions. Keep the fault areas broad at first: the sensor circuit wiring, connector condition, the sensor common circuit, and only after the circuit checks good, the climate control unit itself.
Reproduce the operating condition
Before going into the pin checks, start with the basic troubleshooting approach so you are not chasing this code without the normal preliminary review. Then reproduce the operating condition for the code by turning the vehicle to the ON mode. That is the confirmation setup before moving into the circuit inspection.
Set up the circuit inspection
Next, isolate the circuit before measuring it. Disconnect the in-car temperature sensor 2P connector, then disconnect climate control unit connector A, the 28P connector. From there, perform the TR and SENSOR_COM circuit inspection, starting with the open-circuit checks and then the short-to-power check that follows.
Check the TR open circuit path
With IG Mode OFF, check the TR circuit between the disconnected in-car temperature sensor 2P connector female terminals and the disconnected climate control unit connector A 28P female terminals. A normal TR open-wire inspection is less than 1 Ω. If that check is not normal, treat it as a circuit problem and stay in the wiring diagnosis.
Check the SENSOR_COM open circuit path
Still with IG Mode OFF, check the SENSOR_COM circuit between the disconnected in-car temperature sensor 2P connector female terminals and the disconnected climate control unit connector A 28P female terminals. A normal SENSOR_COM open-wire inspection is less than 1 Ω. If this check is abnormal, the problem is in the circuit, not something to skip past.
Check for TR shorted to power
After the open-circuit checks, move to the short-to-power side of the inspection. With IG Mode ON, inspect the TR circuit for a short to power. The normal result is not shorted. If it is shorted, stay with the circuit fault path.
Make the final call
If the TR and SENSOR_COM inspection items all show normal results, the diagnostic path points to a problem with the remaining climate control unit. Keep that decision after the circuit checks, not before them.
Closing takeaway
Takeaway: for B1225, prove the in-car temperature sensor wiring first, especially the TR and SENSOR_COM paths. If the circuit tests abnormal, stay in the circuit. If the listed circuit checks are normal, the remaining fault area is the climate control unit. For more diagnostic training, visit stepdiagnostics.com.
Final check
This code should be approached as a circuit diagnosis first, with the climate control unit considered only after the specified circuit inspections are normal.
For more guided automotive diagnostics, visit STEP Diagnostics.
