Case IH 3969 – Engine Control Unit (ECU): Invalid Injector Codes / Quick Response definition error

Indicates that the engine ECU memory contains invalid injector calibration codes or has a dataset error. (Note: The description 'Post Drive Relay LS Over Temp' is typically associated with code 1485, indicating the ECU detected an over-temperature condition on the low side of the post drive relay circuit).

• Steiger tractors
• Magnum tractors
• Skid Steer Loaders (SSL)
• Compact Track Loaders (CTL)

1. 1

   Verify the active code using a diagnostic service tool (like CNH EST) to confirm whether the issue is 3969 (Injector Codes) or 1485 (Post Drive Relay).

2. 2

   For the Post Drive Relay Over Temp: Check the ECU main relay and post drive relay for signs of thermal damage or melting.

3. 3

   For the Post Drive Relay Over Temp: Test the low-side circuit wiring between the relay and the ECU for a short to battery or excessive resistance.

4. 4

   For Code 3969 (Invalid Injector Codes): Verify the engine dataset in the ECU memory.

5. 5

   For Code 3969 (Invalid Injector Codes): Read the calibration codes printed on the physical fuel injectors and ensure they match the codes programmed into the Engine ECU.

1. 1

   Disconnect battery

   Disconnect battery power before servicing engine ECU components.

2. 2

   Inspect and replace relay

   If the issue is the Post Drive Relay, locate the engine relay fuse box. Remove and inspect the Post Drive Relay / ECU Main Relay. If the relay is internally shorted or damaged, replace it.

3. 3

   Inspect wiring harness

   Inspect the wiring harness connector at the relay and the ECU for heat damage. Repair any damaged pins.

4. 4

   Re-enter injector codes

   If the issue is Code 3969, connect the CNH Electronic Service Tool (EST). Navigate to the engine programming menu and re-enter the correct injector trim codes for all cylinders.

5. 5

   Clear fault codes

   Clear the fault codes and cycle the key to verify the code does not return.