What this code means
B121D may indicate that the climate control unit is not receiving expected communication from the humidity/in-car temperature sensor.
What the vehicle may do
- The automatic climate control may not respond as expected.
- Cabin temperature or humidity compensation can be affected.
- HVAC performance may feel inconsistent depending on operating conditions.
Possible fault areas
- Possible sensor communication wiring issue
- Possible humidity/in-car temperature sensor issue
- Possible related HVAC component issue
- Possible climate control unit issue after earlier checks are normal
Diagnostic path
Code meaning and starting point
On this 2020 to 2025 Honda CR-V Hybrid, B121D is a climate control code for a humidity and in-car temperature sensor communication problem. In plain terms, the climate control unit may not be getting the data it expects from that sensor. The driver may notice automatic HVAC operation that does not feel quite right, and the possible fault areas can include the sensor communication wiring, the sensor itself, other related HVAC components, and, after the earlier checks are proven good, the climate control unit. Start with the basic system checks, keep the diagnosis structured, then turn the vehicle to the ON mode to confirm the condition.
Set up the circuit inspection
For the circuit inspection, work with the humidity/in-car temperature sensor connector, 4P, disconnected, and climate control unit connector A, 28P, disconnected. The circuits being checked are S_5V, SENSOR_COM, HUM_DATA, and HUM_CLK. At this point, the goal is simple: prove the power, ground reference, and communication paths before moving on to component judgment.
Check the circuits for opens
With IG Mode OFF, check each circuit for an open. S_5V should have less than 1 Ω. SENSOR_COM should have less than 1 Ω. HUM_DATA should have less than 1 Ω. HUM_CLK should have less than 1 Ω. If one of those readings is not normal, stay on the wiring side of the diagnosis instead of moving ahead to the sensor or control unit.
Check for shorts
Next, still with IG Mode OFF, check HUM_DATA and HUM_CLK for a wire-to-wire short. Normal is more than 1 MΩ. Then check for shorts to ground. S_5V should be more than 1 MΩ to ground. HUM_DATA should be more than 1 MΩ to ground. HUM_CLK should be more than 1 MΩ to ground.
Interpret the circuit results
Now evaluate the S_5V, SENSOR_COM, HUM_DATA, and HUM_CLK inspection results as a group. If the circuit inspection is abnormal, the problem is in the circuit. Do not skip past that result; correct the circuit issue before judging the sensor or the climate control unit.
Use a known-good sensor when wiring checks pass
If the wiring inspection does not point to a circuit fault, move to the humidity/in-car temperature sensor inspection. Since this item is checked by comparison instead of a direct stand-alone test, install a known-good humidity/in-car temperature sensor and try to reproduce the failure. If the failure is reproduced, the problem is with other related components. If the symptom or DTC goes away, the humidity/in-car temperature sensor is faulty.
Control unit judgment and takeaway
Only after the other inspection items indicate a normal condition should the remaining climate control unit be judged as the cause. Keep verification separate from the circuit and component checks: once the fault area has been corrected, confirm the original symptom or B121D does not return under the same operating condition. The takeaway is to prove the four circuit paths first, use the known-good sensor check only after that, and do not condemn the climate control unit until the earlier checks are normal. For more diagnostic training, visit stepdiagnostics.com.
Final check
B121D is often approached as a communication-path diagnosis first, then a sensor comparison check, and only then possible control unit judgment.
For more guided automotive diagnostics, visit STEP Diagnostics.
