OBD Guide
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B121D

Rear HVAC Mode Control Input Circuit Short

B121D is an OBD-II diagnostic trouble code meaning: Rear HVAC Mode Control Input Circuit Short. Common causes: short circuit in the rear climate control wiring harness, hvac control unit malfunction. Estimated repair cost: $56–222.

Severity
⚠️ Medium
Can you drive?
Limited driving only, diagnose soon
Approx. repair cost
$56–222 (est.)

Symptoms

  • Rear climate control does not work
  • Incorrect temperature/ventilation speed control in the rear compartment
  • Hot or cold air does not flow to the rear vents

Causes

  • Short circuit in the rear climate control wiring harness
  • HVAC control unit malfunction
  • Damage to solenoid valves or damper actuators
  • Contact corrosion

How to Fix

  1. Conduct system diagnostics using a scanner to confirm the code
  2. Check the integrity of the rear HVAC wiring harness and connections.
  3. Test the functionality of the HVAC control unit
  4. Check resistance and power supply to circuit elements
  5. Replace damaged components

Related codes

B121CB121EB121F

FAQ

What does the B121D code mean?

B121D is an OBD-II diagnostic trouble code that indicates: Rear HVAC Mode Control Input Circuit Short

What causes a B121D error code?

The most common causes of B121D include: Short circuit in the rear climate control wiring harness; HVAC control unit malfunction; Damage to solenoid valves or damper actuators; Contact corrosion.

How do I fix a B121D diagnostic trouble code?

To fix B121D: Conduct system diagnostics using a scanner to confirm the code. Check the integrity of the rear HVAC wiring harness and connections.. Test the functionality of the HVAC control unit. For a complete diagnosis, use an OBD-II scanner.

Is it safe to drive with B121D?

Limited driving only, diagnose soon

How much does it cost to fix B121D?

The estimated repair cost for B121D is $56–222. Actual cost depends on your vehicle, location, and labor rates.

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See also: Russian version · NationStat