What this code means
B121D may mean 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 inaccurate or inconsistent.
- The system may store the code even if the driver does not notice a major symptom.
Possible fault areas
- A possible humidity and in-car temperature sensor issue.
- Possible circuit or connector concerns in the sensor power, return, data, or clock paths.
- A possible climate control unit concern after the other areas test normally.
Diagnostic path
Opening and code logic
On this 2016 to 2024 Honda Civic with the 2.0 gas engine, B121D is a climate control communication fault for the humidity and in-car temperature sensor. In plain terms, the climate control unit may not be receiving the data it expects from that sensor. The driver may notice automatic climate control behavior that seems off, or the system may simply store the code. The broad possibilities are the sensor itself, its power, return, data, or clock circuits, connector issues, or the climate control unit. For this code, the logic is simple: if the climate control unit cannot receive the specified data from the humidity and in-car temperature sensor, it stores B121D.
Start with setup
Start with the basic system checks and follow a structured diagnostic approach before getting deep into the circuit. Then turn the vehicle to the ON mode. To inspect the S_5V, SENSOR_COM, HUM_DATA, and HUM_CLK circuits, disconnect the humidity and in-car temperature sensor 4P connector, and disconnect climate control unit connector A, the 28P connector. Before probing, make sure the connector face and terminal locations are positively identified. If that information is not in front of you, pause and get the correct connector view. Do not guess at pin locations.
Check the open circuits first
With IG Mode OFF, check the open-wire side of the circuit first. Inspect S_5V for an open, and the normal result is less than 1 Ω. Then inspect SENSOR_COM for an open, also looking for less than 1 Ω. Next, check HUM_DATA for an open, again less than 1 Ω. Finally, check HUM_CLK for an open, and that should also be less than 1 Ω. If any of those circuit inspections is abnormal, treat that as a circuit problem before moving on.
Check for shorts
After the open checks, keep IG Mode OFF and check whether HUM_DATA and HUM_CLK are shorted wire-to-wire. The normal result is more than 1 MΩ. Then check the short-to-ground side of the circuit: S_5V should be not shorted, HUM_DATA should be not shorted, and HUM_CLK should be not shorted. If one of these checks fails, stay with the circuit fault instead of condemning the sensor or the control unit.
Use a known-good sensor for the sensor decision
There is no direct sensor inspection in this diagnostic path, so the sensor call is made by substitution. Install a known-good humidity and in-car temperature sensor, then try to reproduce the failure. If the symptom or B121D goes away with the known-good sensor installed, the original humidity and in-car temperature sensor is faulty. If the failure is reproduced with the known-good sensor installed, the problem is in other related components, so continue evaluating the remaining parts of the path instead of stopping at the sensor.
Control unit conclusion and verification
If the circuit checks are normal, the known-good sensor does not clear the failure, and the other inspection items indicate a normal condition, the remaining conclusion is a problem with the climate control unit. After the fault area is corrected, verify the repair by trying to reproduce the same failure condition and confirming B121D stays gone. The takeaway is to prove the wiring first, use the known-good sensor only after the circuit checks, and leave the climate control unit as the final call. For more diagnostic training, visit stepdiagnostics.com.
Final check
This code is often approached by proving the circuits first, then using a known-good sensor to separate a sensor issue from the remaining control-side fault area.
For more guided automotive diagnostics, visit STEP Diagnostics.
