What this code means
B121D may mean the climate control unit is not receiving expected data from the humidity/in-car temperature sensor.
What the vehicle may do
- Climate control operation may feel inconsistent or incorrect.
- Cabin temperature or humidity-related control response can be affected.
Possible fault areas
- A possible humidity/in-car temperature sensor issue.
- A possible wiring or connector issue between the sensor and climate control unit.
- A possible climate control unit issue after other checks are normal.
Diagnostic path
Opening context
On this Civic, B121D is a communication fault between the climate control unit and the humidity/in-car temperature sensor. In plain terms, the control unit may not be getting the data it expects from that sensor. The driver may notice climate control behavior that feels off or inconsistent, and the possible fault areas can include the sensor, the wiring between the sensor and the climate control unit, or the climate control unit itself. Start with the basic system checks, and make sure the diagnostic approach is clear before moving into circuit testing.
Confirm the code path
For the confirmation step, turn the vehicle to the ON mode. The logic behind this code is simple: if the climate control unit cannot receive the specified data from the humidity/in-car temperature sensor, it judges the condition abnormal and stores B121D.
Set up the circuit inspection
Next, move into the sensor communication circuit checks. Disconnect the humidity/in-car temperature sensor 4P connector, and disconnect climate control unit connector A, the 28P connector. You are checking the S_5V, SENSOR_COM, HUM_DATA, and HUM_CLK paths with the circuit isolated.
Check the open circuits
With IG Mode OFF, check each open-wire path in order. Start with S_5V, then SENSOR_COM, then HUM_DATA, and then HUM_CLK. Each one should show less than 1 Ω. If one of those checks is abnormal, treat it as a circuit problem and stay on the wiring side before moving on.
Check for wire-to-wire and ground shorts
Still with IG Mode OFF, check HUM_DATA and HUM_CLK for a short between the two circuits. Normal is more than 1 MΩ. After that, use the short-circuit inspection method and check S_5V, HUM_DATA, and HUM_CLK for shorts to ground. Those ground-short checks should be not shorted. Again, any abnormal result points to a circuit problem.
Use the known-good sensor check
If the circuit checks do not identify the issue, inspect the humidity/in-car temperature sensor by installing a known-good sensor and trying to reproduce the failure. This step matters because there is no direct test method given for that inspection item. If the symptom or DTC goes away with the known-good sensor, the humidity/in-car temperature sensor is faulty. If the failure is still reproduced, look at the other related components instead of condemning the sensor.
Climate control unit decision and verification
Only after the other inspection items have shown normal results does the remaining climate control unit become the cause in this diagnostic path. Keep that decision at the end of the process, after the wiring and known-good sensor checks have been worked through. Then verify the repair and confirm the code stays gone.
Takeaway and CTA
The takeaway: prove the communication circuits first, use the known-good sensor check exactly where the path calls for it, and leave the climate control unit as the remaining suspect only after the earlier checks are normal. For more diagnostic training, visit stepdiagnostics.com.
Final check
B121D is often approached by proving the communication wiring first, then using a known-good sensor check, and only then considering the climate control unit as the remaining cause.
For more guided automotive diagnostics, visit STEP Diagnostics.
