Case IH 121 – ATC: Temperature select pot open/shorted to power

This code indicates a circuit failure related to the temperature selection potentiometer (the dial or switch inside the cab used by the operator to adjust the climate control temperature). The system has detected that the circuit is either completely open (a broken wire or disconnected plug) or shorted directly to a power source.

• STX Steiger
• Magnum series
• Puma
• Steiger
• Maxxum
• Farmall
• Axial-Flow series
• Case Wheel Loaders

1. 1

   Access the back of the cab climate control panel and visually inspect the wiring harness connecting the temperature control dial to the ATC module for chafing, pinched wires, or breaks.

2. 2

   Disconnect the plug at the potentiometer and the ATC module to check for backed-out pins, loose connections, or corrosion.

3. 3

   Using a multimeter, test the resistance of the temperature select potentiometer while turning the dial. The resistance should sweep smoothly without dropping out (some are rated at 50,000 Ohms).

4. 4

   Test the signal wire between the potentiometer and the ATC module for continuity.

5. 5

   Test the signal wire for a short to voltage, which would trigger the 'shorted to power' condition.

1. 1

   Disconnect Power

   Turn off the machine's ignition and disconnect the negative battery cable to prevent electrical damage.

2. 2

   Access the Control Panel

   Remove the necessary cab trim, bezels, or console covers to access the rear of the ATC / HVAC control panel.

3. 3

   Remove the Faulty Component

   Disconnect the electrical harness from the temperature select potentiometer. Remove the retaining nut, clips, or screws holding the potentiometer to the panel, and pull the switch out.

4. 4

   Install the New Part

   Insert the replacement potentiometer into the control panel and secure it with the retaining hardware.

5. 5

   Reconnect

   Plug the electrical harness back into the new potentiometer. Ensure the connector clicks into place securely.

6. 6

   Reassemble

   Reinstall all cab trim and console panels.

7. 7

   Test and Clear Codes

   Reconnect the battery. Turn the ignition on, clear the stored ATC 121 fault code using the machine's diagnostic display, and test the climate control system to ensure the temperature adjusts correctly and the code does not return.