GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 Yukon - 2WD 🢒 HVAC 🢒 HVAC Systems - Automatic 🢒 DTC B1020 Auxiliary ECU Malfunction

Schematic Reference HVAC Air Delivery/Temperature Control Schematics

Circuit Description

This DTC is an indication of a malfunction with the Infrared thermistor which is internal to the rear auxiliary control module.

Conditions for Running the DTC

The ignition is turned ON.

Conditions for Setting the DTC
    • This test checks the value of the Infrared thermistor (part of the Rear Aux ECC module) and the stored thermistor values. If the stored values are above or below the system specifications, the DTC will be set.
    • The above conditions are present for 3 consecutive failures.

Action Taken When the DTC Sets
    • The automatic portion of the heater and A/C system will be disabled.
    • A calibrated (failsoft) value of 75 degrees will be used in all HVAC calculations requiring the temperature of the rear passenger compartment.

Conditions for Clearing the DTC
    • The DTC will become history if the HVAC Module no longer detects a failure.
    • The history DTC will clear after 50 fault free ignition cycles.
    • The DTC can be cleared with a scan tool.

Step

Action

Yes

No

1

Did you perform the HVAC Diagnostic System Check?

Go to Step 2

Go to Diagnostic System Check

2
  1. Use the scan tool to clear the DTCs.
  2. Turn OFF the ignition and wait 45 seconds.
  3. Turn ON the ignition with the engine OFF.
  4. Select the HVAC DTC display function on the scan tool.

Does the scan tool display DTC B1020?

Go to Step 3

Go to Testing for Intermittent Conditions and Poor Connections

3

Test the auxiliary mode door control circuit between the front auxiliary control module and the rear auxiliary control module for a short to voltage. Refer to and in wiring systems.

Did you find and correct the condition?

Go to Step 6

Go to Step 4

4

Test the auxiliary temperature door control circuit between the front auxiliary control module and the rear auxiliary control module for a short to voltage. Refer to and in wiring systems.

Did you find and correct the condition?

Go to Step 6

Go to Step 5

5

Replace the Rear Aux (ECC ) control module. Refer to Control Assembly Replacement - Auxiliary .

Is the repair complete?

Go to Step 6

--

6
  1. Use the scan tool in order to clear the DTCs
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

Go to Step 2

System OK