Case IH 19678 – DCU (Dosing Control Unit): Ambient Air Temperature Sensor Range/Performance

The control module is receiving an invalid, out-of-range, or missing ambient temperature signal over the J1939 CAN bus network, or the network is being flooded with too many CAN messages regarding this sensor's data.

• Tier 4a
• Case IH Magnum Series: 235, 260, 290, 315, 340 (Series 4)
• Case IH STX Steiger Series: 350 to 600 HP models

1. 1

   Visually inspect the Ambient Air Temperature Sensor housing (typically mounted near the front grille, air intake, or air filter housing) for physical damage.

2. 2

   Disconnect the sensor and inspect the connector and harness pins for corrosion, moisture ingress, or pushed-back terminals.

3. 3

   Check the J1939 CAN bus wiring between the sensor/Engine ECU and the DCU for shorts or high resistance.

4. 4

   Look for other active CAN bus communication codes. If multiple sensors are dropping offline, the issue is likely a damaged CAN backbone harness rather than the temperature sensor itself.

5. 5

   Use a diagnostic scan tool to read the live ambient air temperature value. If it reads an extreme default value (e.g., -40°C or maximum temperature) or fluctuates wildly, the sensor has likely failed internally.

1. 1

   Power Down

   Turn off the ignition and disconnect the negative battery cable to prevent electrical spikes on the CAN bus.

2. 2

   Remove the Sensor

   Disconnect the electrical harness from the Ambient Air Temperature Sensor. Unclip or unbolt the sensor from its mounting bracket on the intake/grille area.

3. 3

   Clean the Connection

   Spray the harness connector with electronic contact cleaner and ensure it is completely dry. Apply a small amount of dielectric grease to the connector seal to prevent future moisture ingress.

4. 4

   Install the New Sensor

   Mount the replacement sensor into the bracket and plug in the electrical connector until it clicks securely.

5. 5

   Re-establish Power

   Reconnect the negative battery cable.

6. 6

   Clear Codes

   Turn the ignition to the ON position (engine off). Connect a diagnostic scan tool to the tractor's diagnostic port and clear the 19678 / P0071 fault code from the DCU.

7. 7

   Test Operation

   Start the engine and monitor the live ambient temperature data on the dash or scan tool to ensure it matches the actual outside temperature and the code does not return.