John Deere 1548.04 – ATC (Automatic Temperature Control): Outlet Air Temperature Sensor Circuit Voltage Low

The controller has detected that the signal voltage from the vent outlet air temperature sensor (Sending Unit B128) has dropped below 0.1 VDC (or below 0 volts). This condition typically means that the sensor's internal resistance has dropped below 50 Ohms (which the system interprets as an impossibly high temperature of 293°F / 145°C), or there is a direct short to ground in the sensor's wiring circuit. When this code is active, the controller restricts automatic temperature control, and the water valve is adjusted based only on the difference between the actual cab temperature and the desired temperature.

• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 6020 Series (6120-6520)
• 7015/7020 Series (7715, 7815, 7720, 7820, 7920)
• 8020/8030 Series (8120-8530)
• 9030 Series (9230-9630)
• 9470RT
• 9520RT
• 9570RT
• WTS Series (9540-9680)
• STS Series (9560-9870)

1. 1

   Verify the Code: Restart the engine to see if the code is active or stored. Sometimes communication glitches can trigger a false code.

2. 2

   Locate the Sensor: Find the Outlet Air Temperature Sensor (Component B128) on the HVAC unit.

3. 3

   Inspect the Harness: Disconnect the 2-pin plug (X601) at the sensor. Inspect the connector for corrosion, moisture, or bent pins.

4. 4

   Test for a Short to Ground: With the sensor unplugged, turn the key ON and check the voltage at the harness side. If the code changes to 001548.03 (Voltage High / Open Circuit), the wiring is fine, and the sensor itself is internally shorted. If the code remains 001548.04, the wiring harness is shorted to ground somewhere between the sensor and the ATC controller.

5. 5

   Test Sensor Resistance: Measure the resistance across the two pins of the sensor. If the multimeter reads less than 50 Ohms, the sensor has failed and must be replaced.

1. 1

   Turn off the engine, remove the key, and allow the machine to cool down.

2. 2

   Access the HVAC blower/evaporator housing (location varies by cab model, often under the seat or in the roof) to reach the B128 Outlet Air Temperature Sensor.

3. 3

   If the wiring is faulty: Trace the harness back to the ATC module. Locate the chafed wire that is shorting to the chassis ground and repair it using heat-shrink connectors, or replace that section of the harness.

4. 4

   If the sensor is faulty: Unplug the 2-pin X601 connector. Unthread or unclip the faulty sensor from the air duct/housing. Install the new sensor. (Be sure to lubricate and install a new O-ring if the sensor threads into a sealed duct area). Reconnect the 2-pin harness.

5. 5

   Turn the key ON and use the CommandCenter or Service Advisor to clear the stored fault codes.

6. 6

   Run the ClimaTrak system to verify that the A/C and heater blend properly and the code does not return.