What this code means
B1231 may indicate that the climate control unit is seeing an open circuit condition for the evaporator temperature sensor signal.
What the vehicle may do
- The air conditioning may not control evaporator temperature as expected.
- The climate control system may store the fault with limited driver-noticeable symptoms.
- Cabin cooling behavior can be inconsistent depending on how the system responds to the missing sensor signal.
Possible fault areas
- A possible evaporator temperature sensor fault.
- Possible loose connections or poor terminal contact in the evaporator temperature sensor circuit.
- A possible open in the wiring between the evaporator temperature sensor and the climate control unit.
- A possible climate control unit fault after the sensor and wiring have checked OK.
Diagnostic path
Opening context
On this Civic, B1231 points to an evaporator temperature sensor circuit open in the climate control system. In plain terms, the climate control unit may not be seeing a valid evaporator temperature signal. The driver may notice air conditioning that does not control as expected, or the system may simply store the fault with limited symptoms. The possible fault areas are the evaporator temperature sensor, the sensor circuit wiring and connectors, or the climate control unit itself. The goal here is to confirm the fault, separate a bad sensor from an open circuit, and only then decide whether the control unit is still suspect.
Confirm the fault first
Start by proving whether the fault is current. Clear 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 DTCs. If B1231, or the equivalent climate control self-diagnostic indication, comes back, move on to the evaporator temperature sensor check. If it does not come back, treat it as an intermittent failure and check for loose wires or poor connections in the evaporator temperature sensor circuit.
Check the evaporator temperature sensor
Next, turn the ignition switch to LOCK (0). Remove the evaporator temperature sensor and test it. If the sensor does not test OK, replace the evaporator temperature sensor. If the sensor tests OK, leave the evaporator temperature sensor disconnected and continue into the wiring checks.
Check the TEVA circuit for an open
Now check the TEVA wire between the climate control unit and the evaporator temperature sensor. Disconnect the climate control unit 32P connector. With IG LOCK (0), the climate control unit 32P connector disconnected, and the evaporator temperature sensor 2P connector disconnected, check continuity on the TEVA circuit. Test between the climate control unit 32P connector No. 14 (BRN) and the evaporator temperature sensor 2P connector No. 1 (BRN). If there is continuity, the TEVA wire is OK and you move to the sensor common circuit. If there is no continuity, repair the open in the wire between the climate control unit and the evaporator temperature sensor.
Check the sensor common circuit
The last circuit check is the SENSOR_COM/SENS-COM side. Keep the same test condition: IG LOCK (0), the climate control unit 32P connector disconnected, and the evaporator temperature sensor 2P connector disconnected. Check continuity between the climate control unit 32P connector No. 20 (RED) and the evaporator temperature sensor 2P connector No. 2 (RED). If there is no continuity, repair the open in the wire between the climate control unit and the evaporator temperature sensor. If there is continuity, the SENSOR_COM/SENS-COM wires are OK, and the diagnostic path calls for replacing the climate control unit.
Takeaway
The clean way through B1231 is simple: confirm the fault is current, test the evaporator temperature sensor, then check the two circuit legs for opens before condemning the control unit. For more diagnostic training, visit stepdiagnostics.com.
Final check
B1231 diagnosis often comes down to proving whether the sensor, the wiring, or the climate control unit is responsible for the open-circuit reading.
For more guided automotive diagnostics, visit STEP Diagnostics.
