DTC Troubleshooting: B121B (H and A/C ON)
  1. Turn the ignition switch to ON (II), and measure the voltage between heater control unit connector A (28P) No. 11 terminal and body ground.
Is there any voltage?
YES -
Repair short to power in the wire between the heater control unit and the mode control motor. This short may also damage the heater control unit. Repair short to power before replacing the heater control unit.■
NO -
Substitute a known-good heater control unit, and recheck. If the symptom/indication goes away, replace the original heater control unit .■
  1. Turn the ignition switch to LOCK (0), and disconnect heater control unit connector B (12P).
  1. Disconnect these items:
    • Driver's air mix control motor
    • Passenger's air mix control motor
    • Recirculation control motor
    • Mode control motor