What this code means
This climate control code means the control unit may be seeing an open in the evaporator temperature sensor circuit.
What the vehicle may do
- The A/C system may operate inconsistently.
- Cabin cooling can seem abnormal if the climate control unit is not getting a usable evaporator temperature signal.
- The fault may be intermittent if the code does not return during verification.
Possible fault areas
- Possible loose wiring or poor connector contact in the evaporator temperature sensor circuit.
- Possible open or shorted wiring in the sensor circuit.
- Possible evaporator temperature sensor fault.
- Possible climate control unit fault after the wiring and sensor checks pass.
Diagnostic path
Open and code meaning
On this 2010 to 2017 Honda Accord with the 2.4 gas engine, B1231 is a climate control unit code for a possible open in the evaporator temperature sensor circuit. In plain terms, the climate control unit may not be getting the evaporator temperature signal it expects. The driver may notice A/C operation that seems inconsistent, limited, or not responding normally, and the broad fault areas can include the evaporator temperature sensor circuit, connector condition, the sensor itself, or, after the circuit and sensor check out, the climate control unit. The diagnostic path here is straightforward: first prove the code is current, then check the two sensor circuit legs, then check for a short between the involved circuits, and only then evaluate the sensor and control unit decision.
Verify the code is current
Start by verifying the problem. Clear the DTC with the HDS. Then turn the ignition switch to LOCK (0) and back to ON (II), or press the engine start/stop button to select OFF mode and then ON mode. Run the self-diagnostic function with the HDS or with the climate control unit, then check for DTCs. If B1231, or indicator 09, is indicated again, continue with the circuit checks. If it is not indicated, treat it as an intermittent failure and check for loose wires or poor connections on the evaporator temperature sensor circuit.
Check the TEVA line for an open
Next, check the TEVA line for an open. With the ignition switch at LOCK (0), or the engine start/stop button in OFF mode, disconnect the climate control unit 32P connector and the evaporator temperature sensor 2P connector. Check continuity on the TEVA circuit between climate control unit 32P connector No. 26, RED, and evaporator temperature sensor 2P connector No. 1, BRN. If there is continuity, the TEVA wire is not open, so 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 SENSOR COM or SENS-COM for an open
Now check the SENSOR COM, also called SENS-COM, side of the circuit. Keep the ignition in LOCK (0) or OFF mode, with the climate control unit 32P connector and evaporator temperature sensor 2P connector still disconnected. Check continuity between climate control unit 32P connector No. 22, BLU, and evaporator temperature sensor 2P connector No. 2, RED. If there is continuity, that wire is OK and the next step is the short check. If there is no continuity, repair the open in the wire between the climate control unit and the evaporator temperature sensor.
Check for a short between TEVA and S 5V
With the same connectors still disconnected and the ignition still at LOCK (0) or OFF mode, check for a short between the TEVA and S 5V circuits. Check continuity between climate control unit 32P connector No. 24, BLK, and climate control unit 32P connector No. 26, RED. If there is continuity, repair the short in the wires between the climate control unit and the evaporator temperature sensor. If there is no continuity, the TEVA wire and the S 5V, or S5V, wire are OK, so continue to the evaporator temperature sensor check.
Check the evaporator temperature sensor and make the final call
At this point, test the evaporator temperature sensor. If the evaporator temperature sensor tests OK, the diagnostic path ends with replacing the climate control unit. If the evaporator temperature sensor does not test OK, replace the evaporator temperature sensor. Keep that order: prove the code is current, prove the wiring, check for the short, and then make the sensor-versus-control-unit decision.
Takeaway and CTA
The takeaway on B1231 is to avoid jumping straight to a part. This code is handled by proving the fault is present, checking the evaporator temperature sensor circuit for opens, checking the short path, and then using the sensor test result to decide the final repair. For more diagnostic training, visit stepdiagnostics.com.
Final check
The clean approach is to confirm the code, isolate the evaporator temperature sensor wiring, check for a short, and then use the sensor test result before making the final repair decision.
For more guided automotive diagnostics, visit STEP Diagnostics.
