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Pinpoint Test D: DTC B1255 Or B1257

WARNING: This page is about a different car, the 2010 Lincoln MKX and 2010 Ford Edge. However, it is still accessible from the selected car via links, so may be relevant.
  1. D1 CHECK THE SENSOR RESISTANCE 
    • Ignition OFF.
    • Disconnect: Ambient Temperature Sensor C198.
    • Carry out the Ambient Temperature Sensor component test. See Component Tests .
    • Does the ambient temperature sensor test OK? 
    • Yes:  GO to D2  .
    • No:  INSTALL a new ambient temperature sensor. CLEAR the DTCs. REPEAT the self-test. TEST the system for normal operation.
  2. D2 CHECK THE HVAC SENSOR OUTPUT VOLTAGE 
    • Ignition ON.
    • Press the AUTO button.
    • Measure the voltage between ambient temperature sensor C198-2, circuit VH407 (YE/GN), harness side and C198-1, circuit RH111 (GY/BU), harness side.
      Fig 1: Measuring Voltage Between Ambient Temperature Sensor C198-2, Circuit VH407 (YE/GN) And C198-1, Circuit RH111 (GY/BU), Harness Side
      G06505287Courtesy of FORD MOTOR CO.
    • Is the voltage between 4.7 and 5.1 volts? 
    • Yes:  INSTALL a new ambient temperature sensor. CLEAR the DTCs. REPEAT the self-test. If code returns, GO to  D7.
    • No: For DTC B1255  , GO to D3  .

      For DTC B1257  , GO to  D5.

  3. D3 CHECK CIRCUIT VH407 (YE/GN) FOR AN OPEN 
    • Ignition OFF.
    • Disconnect: HVAC Module - DATC  C228A.
    • Measure the resistance between HVAC module - DATC  C228A-2, circuit VH407 (YE/GN), harness side and ambient temperature sensor C198-2, circuit VH407 (YE/GN), harness side.
      Fig 2: Measuring Resistance Between HVAC Module - DATC C228A-2, Circuit VH407 (YE/GN) And Ambient Temperature Sensor C198-2, Circuit VH407 (YE/GN)
      G06505288Courtesy of FORD MOTOR CO.
    • Is the resistance less than 5 ohms? 
    • Yes:  GO to D4  .
    • No:  REPAIR circuit VH407 (YE/GN) for an open. CLEAR the DTCs. REPEAT the self-test. TEST the system for normal operation.
  4. D4 CHECK CIRCUIT RH111 (GY/BU) FOR AN OPEN 
    • Measure the resistance between HVAC module - DATC  C228A-10, circuit RH111 (GY/BU), harness side and ambient temperature sensor C198-1, circuit RH111 (GY/BU), harness side.
      Fig 3: Measuring Resistance Between HVAC Module - DATC C228A-10, Circuit RH111 (GY/BU) And Ambient Temperature Sensor C198-1, Circuit RH111 (GY/BU)
      G06505289Courtesy of FORD MOTOR CO.
    • Is the resistance less than 5 ohms? 
    • Yes:  GO to  D7.
    • No:  REPAIR circuit RH111 (GY/BU) for an open. CLEAR the DTCs. REPEAT the self-test. TEST the system for normal operation.
  5. D5 CHECK CIRCUIT VH407 (YE/GN) FOR A SHORT TO CIRCUIT RH111 (GY/BU) 
    • Ignition OFF.
    • Disconnect: HVAC Module - DATC  C228A.
    • Measure the resistance between ambient temperature sensor C198-2, circuit VH407 (YE/GN), harness side and C198-1, circuit RH111 (GY/BU), harness side.
      Fig 4: Measuring Resistance Between Ambient Temperature Sensor C198-2, Circuit VH407 (YE/GN) And C198-1, Circuit RH111 (GY/BU)
      G06505290Courtesy of FORD MOTOR CO.
    • Is the resistance greater than 10,000 ohms? 
    • Yes:  GO to D6  .
    • No:  REPAIR circuit VH407 (YE/GN) for a short to circuit RH111 (GY/BU). CLEAR the DTCs. REPEAT the self-test. TEST the system for normal operation.
  6. D6 CHECK CIRCUIT VH407 (YE/GN) FOR A SHORT TO GROUND 
    • Measure the resistance between HVAC module - DATC  C228A-2, circuit VH407 (YE/GN), harness side and ground.
      Fig 5: Measuring Resistance Between HVAC Module - DATC C228A-2, Circuit Vh407 (YE/GN), Harness Side And Ground
      G06064539Courtesy of FORD MOTOR CO.
    • Is the resistance greater than 10,000 ohms? 
    • Yes:  GO to D7  .
    • No:  REPAIR circuit VH407 (YE/GN) for a short to ground. CLEAR the DTCs. REPEAT the self-test. TEST the system for normal operation.
  7. D7 CHECK THE HVAC MODULE CONNECTION 
    • Inspect the HVAC module connectors for:
      • corrosion.
      • pushed-out terminals.
      • damaged terminals.
    • Connect and correctly seat all the HVAC module connectors.
    • Clear the DTCs.
    • Operate the system and verify the concern is still present.
    • Is the concern still present? 
    • Yes:  INSTALL a new HVAC module. REFER to CLIMATE CONTROL . TEST the system for normal operation.
    • No:  The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector. CLEAR the DTCs. REPEAT the self-test. TEST the system for normal operation.
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