Case IH 16121 – ATC (Automatic Temperature Control): Temperature select potentiometer is open or shorted to power

The climate control system is receiving an abnormal voltage signal from the cab's temperature selection dial. The system has detected either an open circuit (a broken wire, disconnected plug, or failed internal resistor) or a short circuit to the positive power supply.

• Puma/Mid Range Magnum
• Maxxum
• Case IH Magnum Series
• Case IH Puma Series
• Case IH Steiger Series

1. 1

   Inspect the temperature control dial on the cab climate control panel for physical damage or looseness.

2. 2

   Access the back of the climate control panel and check the wiring harness connectors (such as X751 and X755) between the temperature select potentiometer and the ATC module for loose pins or corrosion.

3. 3

   Use a multimeter to test the potentiometer circuit for continuity to rule out an open circuit (broken wire).

4. 4

   Test for voltage on the signal wire with the sensor unplugged to rule out a short to the power supply.

5. 5

   Test the resistance (Ohms) of the potentiometer itself while turning the dial to see if it provides a smooth, linear change in resistance or if it drops out/reads open.

1. 1

   Turn off the tractor and disconnect the battery to prevent further electrical shorts.

2. 2

   Remove the interior cab trim/bezel surrounding the climate control panel to access the HVAC switches.

3. 3

   Disconnect the wiring harness from the back of the temperature select potentiometer.

4. 4

   If the wiring harness showed a short or open circuit during diagnostics, trace the harness back to the ATC module and repair the damaged section of the wire.

5. 5

   If the potentiometer itself failed the multimeter test, remove the retaining nut or clips holding the switch in the panel and remove the faulty switch.

6. 6

   Install the replacement potentiometer/temperature control switch into the panel.

7. 7

   Reconnect the wiring harness and reassemble the cab trim pieces.

8. 8

   Reconnect the battery, turn the key on, and verify that the ATC 121 / 16121 code has cleared and the temperature blend door responds to dial inputs.