Case IH 118 – ATC (Automatic Temperature Control): Outside air sensor shorted to ground

The system has detected that the signal voltage from the outside air temperature sensor (ambient air temperature sensor) has dropped below the acceptable threshold, indicating a direct short to ground. This means the signal wire is either touching a ground source (like the machine's chassis) due to wire damage, or the sensor itself has failed and shorted internally.

• Case IH Magnum 235 - 340 (Series 4)
• Case IH Puma Series (140, 165, 180, 195, 210, 225)
• Case IH Steiger / STX Series

1. 1

   Visually inspect the outside air temperature sensor (typically mounted near the cab fresh air intake or at the front of the cab roof), connector, and wiring harness for physical damage, moisture intrusion, or chafing against the frame.

2. 2

   Perform a disconnect test: Disconnect the sensor's electrical connector, turn the key ON, and check the active fault codes. If the code changes from 118 to 117, the wiring is intact and the sensor itself is internally shorted.

3. 3

   If code 118 remains active while the sensor is unplugged (indicating a wiring short), use a digital multimeter to test the signal wire on the harness side for continuity to ground. Trace the harness back to the ATC module, looking for pinched wires or rubbed-through insulation.

1. 1

   Power Down

   Turn off the machine, remove the key, and disconnect the battery ground cable to prevent electrical spikes.

2. 2

   Access the Sensor

   Locate the outside air sensor based on your specific model's service manual (usually near the cab air filter housing or roof intake).

3. 3

   Address the Fault

   If the sensor is faulty: Unbolt or unclip the old sensor. Plug the new sensor into the harness and secure it in the factory mounting location. If the wiring is faulty: Repair the chafed/shorted section of the wire using weather-sealed heat shrink connectors, or replace that section of the cab harness. Secure the harness with zip ties to prevent future rubbing.

4. 4

   Reassembly & Testing

   Reconnect the battery. Turn the ignition ON and navigate to the ATC diagnostics screen. Clear the stored fault codes.

5. 5

   Verify Operation

   Run the HVAC system and verify that the ATC module is reading a plausible outside ambient temperature and that Code 118 does not return.