Pinpoint Test B: DTC B1251 Or DTC B1253 - A/C In-Vehicle Temperature Sensor Open Circuit Or Short To Ground

WARNING: This page is about a different car, the 2005 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.

B1 CHECK THE SENSOR RESISTANCE

Key in OFF position.
Disconnect: In-Vehicle Temperature Sensor C233.
Measure the resistance between the in-vehicle temperature sensor pins, component side.

Fig 1: Measuring Resistance Between In-Vehicle Temperature Sensor Pins, Component Side
Courtesy of FORD MOTOR CO.

Note the ambient temperature and compare the in-vehicle temperature sensor resistance with the table below.

IN-VEHICLE SENSOR RESISTANCE SPECIFICATIONS

Temperature	Resistance
10-20°C (50-68°F)	37,000-58,000 ohms
20-30°C (68-86°F)	24,000-37,000 ohms
30-40°C (86-104°F)	16,000-24,000 ohms

Is the resistance within the specified values for the temperature range?

Yes : GO to B2.
No : INSTALL a new in-vehicle temperature sensor. CLEAR the DTCs. TEST the system for normal operation.

B2 CHECK THE DATC MODULE OUTPUT VOLTAGE
- Key in ON position.
- Press the AUTO button on the DATC module.
- Measure the voltage between the in-vehicle temperature sensor C233, circuits 790 (WH/OG) and 470 (PK/BK).
  
  Fig 2: Measuring Voltage Between In-Vehicle Temperature Sensor C233, Circuits 790 (WH/OG) And 470 (PK/BK)
  Courtesy of FORD MOTOR CO.
- Is the voltage between 4.7 and 5.1 volts?
- Yes : INSTALL a new in-vehicle temperature sensor. TEST the system for normal operation. If the code returns, INSTALL a new DATC module. TEST the system for normal operation.
- No : If diagnosing DTC B1251, GO to B3. If diagnosing DTC B1253, GO to B5.
B3 CHECK CIRCUIT 790 (WH/OG) FOR AN OPEN
- Key in OFF position.
- Disconnect: DATC Module C228b.
- Measure the resistance between the DATC module C228b-19, circuit 790 (WH/OG) and the in-vehicle temperature sensor C233, circuit 790 (WH/OG).
  
  Fig 3: Measuring Resistance Between DATC Module C228b-19 And In-Vehicle Temperature Sensor C233
  Courtesy of FORD MOTOR CO.
- Is the resistance less than 5 ohms?
- Yes : GO to B4.
- No : REPAIR circuit 790 (WH/OG). CLEAR the DTCs. TEST the system for normal operation.
B4 CHECK CIRCUIT 470 (PK/BK) FOR AN OPEN
- Measure the resistance between the DATC module C228b-16, circuit 470 (PK/BK) and the in-vehicle temperature sensor C233, circuit 470 (PK/BK).
  
  Fig 4: Measuring Resistance Between DATC Module C228b-16 And In-Vehicle Temperature Sensor C233, Circuit 470 (PK/BK)
  Courtesy of FORD MOTOR CO.
- Is the resistance less than 5 ohms?
- Yes : INSTALL a new DATC module. TEST the system for normal operation.
- No : REPAIR circuit 470 (PK/BK). TEST the system for normal operation.
B5 CHECK CIRCUIT 790 (WH/OG) FOR A SHORT TO CIRCUIT 470 (PK/BK)
- Key in OFF position.
- Disconnect: DATC Module C228b.
- Measure the resistance between the in-vehicle temperature sensor C233, circuit 790 (WH/OG) and circuit 470 (PK/BK).
  
  Fig 5: Measuring Resistance Between In-Vehicle Temperature Sensor C233, Circuit 790 (WH/OG) And Circuit 470 (PK/BK)
  Courtesy of FORD MOTOR CO.
- Is the resistance greater than 10,000 ohms?
- Yes : GO to B6.
- No : REPAIR circuit 790 (WH/OG) for a short to circuit 470 (PK/BK). TEST the system for normal operation.
B6 CHECK CIRCUIT 790 (WH/OG) FOR A SHORT TO GROUND
- Measure the resistance between the DATC module C228b-19, circuit 790 (WH/OG) and ground.
  
  Fig 6: Measuring Resistance Between DATC Module C228b-19, Circuit 790 (WH/OG) And Ground
  Courtesy of FORD MOTOR CO.
- Is the resistance greater than 10,000 ohms?
- Yes : INSTALL a new DATC module. TEST the system for normal operation.
- No : REPAIR circuit 790 (WH/OG) for a short to ground. TEST the system for normal operation.