For 1990-2009 cars

### Circuit Description

The instrument panel module (IPM) sends cruise control data to the engine control module (ECM). The data is sent to the ECM through a communication network called the GM local area network (LAN). Two circuits are used to communicate LAN data between the ECM and IPM. A fault in the LAN will not cause DTC U0423 to set by itself. If a LAN fault occurs, other DTCs will set in addition to DTC U0423.

If the ECM receives invalid cruise control data from the IPM, this DTC sets.

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC U0423 IPM Communication Circuit Performance

### Conditions for Running the DTC

• | The ignition is on for more than 3 seconds.

• | The ignition 1 voltage is between 10.5-18 volts.

• | No other communication errors are present.

• | DTC U0423 runs continuously once the above conditions are met.

### Conditions for Setting the DTC

The ECM receives no valid cruise control data from the IPM for more than 1 second.

### Action Taken When the DTC Sets

• | The control module stores the DTC information into memory when the diagnostic runs and fails.

• | The malfunction indicator lamp (MIL) will not illuminate.

• | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.

• | The driver information center, if equipped, may display a message.

### Conditions for Clearing the DTC

• | A current DTC Last Test Failed clears when the diagnostic runs and passes.

• | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.

• | Clear the DTC with a scan tool.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 |

1. Turn ON the ignition, with the engine OFF.

Important: Record all Failure Records before clearing the DTC. Using Clear Info erases the Failure Records from the engine control module (ECM).

2. Record the DTC Freeze Frame/Failure Records.

Did you record any Freeze Frame/Failure Records for a DTC other than U0423?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3 |

1. Clear the DTC Information with a scan tool.
2. Turn the ignition OFF for at least 30 seconds.
3. Start, and allow the engine to idle.

Did DTC U0423 fail this ignition?

Go to Step 4

Go to Intermittent Conditions

4 | Did an instrument panel module (IPM) DTC, a cruise control related DTC, or any other "U" DTC also set? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 5
5 | Test the LAN circuits in the wiring harness between IPM and the ECM for a high resistance or for a poor connection at the module connectors. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 7 | Go to Step 6
6 | Replace the ECM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 7 | --
7 |

1. Clear the DTCs with a scan tool.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 7

8 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | System OK