The HVAC module sees an open or short on the rear auxiliary mode control input circuit
What it means
B121D. The HVAC module sees an open or short on the rear auxiliary mode control input circuit. The rear airflow mode may not respond..
Findings, check these first
- Faulty rear HVAC mode control
- Rear HVAC mode control harness open or shorted
- Poor electrical connection in the control circuit
- Faulty HVAC module
Standard procedure
- Confirm the code and test the rear mode control circuit
- Inspect the rear auxiliary control and its connector
- Check the input circuit for an open or short
- Verify the HVAC module power and ground
- Replace the failed control or module after tests
Code lookup
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Sources and review
Review: qualified. Read the source limits below.
- B121D FORD Code: Meaning, Causes, and Symptoms | OBD2 Codes - OBD2 Codes
live reference page (obd2-codes.co)
Supports: FORD and Ford-family meaning and likely causes for B121D
Fetched directly. B121D reads "Rear HVAC Mode Control Input Circuit Short". obdii.pro carries no definition for this code, so there is no Ford-family corroboration.
Last updated 2026-10-02.
Report a correction for B121D. Include your vehicle, the issue and a source if you have one.