What this code means
B121D may indicate that the climate control unit is not receiving the expected information from the humidity and in-car temperature sensor.
What the vehicle may do
- Automatic climate control operation may seem incorrect or inconsistent.
- The vehicle may store the code without an obvious driver complaint.
Possible fault areas
- Possible humidity and in-car temperature sensor concern.
- Possible wiring or connector issue in the sensor communication circuits.
- Possible related component concern.
- Possible climate control unit concern after the other checks are normal.
Diagnostic path
Set up the B121D diagnostic path
On this CR-V, B121D is a climate control communication code. In plain terms, the climate control unit may not be getting the data it expects from the humidity and in-car temperature sensor. The customer may notice automatic HVAC behavior that seems off, or there may simply be a stored code. Broadly, think in terms of the sensor, its power feed, common, data, and clock circuits, other related components, and only after the checks are normal, the climate control unit itself. Before troubleshooting, review the overall troubleshooting approach for this system, then turn the vehicle to the ON mode before moving into the circuit testing.
Check the sensor communication circuits first
For the circuit inspection, shut the ignition mode back OFF before measuring. Disconnect the humidity and in-car temperature sensor 4P connector, and disconnect climate control unit connector A, the 28P connector. The circuits being checked are S_5V, SENSOR_COM, HUM_DATA, and HUM_CLK. Work them in order and do not jump ahead to a component call until the wiring checks are done.
Open-circuit checks
With IG Mode OFF, check the S_5V circuit for an open. A normal result is less than 1 Ω. Then check SENSOR_COM for an open, also looking for less than 1 Ω. Next check HUM_DATA for an open, again less than 1 Ω. Last in this group, check HUM_CLK for an open, with the same normal result: less than 1 Ω. If any of these are abnormal, treat that as a circuit problem and stay in the wiring diagnosis.
Short checks
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 in the same diagnostic path: S_5V should be more than 1 MΩ, HUM_DATA should be more than 1 MΩ, and HUM_CLK should be more than 1 MΩ. Any abnormal result here means there is a problem in the circuit, so stay with that circuit fault before condemning a module or sensor.
Use a known-good sensor when the wiring checks pass
There is no direct test for the humidity and in-car temperature sensor in this path, so the check is done with a known-good sensor. Install the known-good humidity and in-car temperature sensor and try to reproduce the failure. If the failure is reproduced, the path points to another related component. If the symptom or DTC goes away with the known-good sensor installed, the original humidity and in-car temperature sensor is faulty.
When the other checks are normal
If the circuit inspections are normal, and the other inspection items point to a normal condition, the remaining fault area is the climate control unit. Keep the logic clean: prove the circuits first, use the known-good sensor check because there is no direct sensor test here, and only then make the call on the control unit.
Takeaway
The key with B121D is not to guess at the sensor or the climate control unit. Verify the four communication circuits first, use the known-good sensor check correctly, and let the results point you to the fault area. For more diagnostic training, visit stepdiagnostics.com.
Final check
This diagnostic path often comes down to proving the communication circuits first, then using a known-good sensor check before judging the climate control unit.
For more guided automotive diagnostics, visit STEP Diagnostics.
