For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptors

DTC U1016: Lost Communication With ECM/PCM
DTC U1017: Lost Communication With Engine Control Module
DTC U1024: Lost Communication With Transmission Control Module
DTC U1026: Lost Communication With ATC/TCCM
DTC U1040: Lost Communication With Electronic Brake Control Module
DTC U1041: Lost Communication With Electronic Brake Control Module
DTC U1064: Lost Communication With Body Control Module (BCM)
DTC U1088: Lost Communication With Sensing and Diagnostic Module (SDM)
DTC U1096: Loss of IPC Communications
DTC U1097: Loss of DIC Communications
DTC U1098: Loss of HUD Communications
DTC U1099: Loss of HUD Communications
DTC U1111: Lost Communication With Chime Module
DTC U1128: Loss of IRC Communications
DTC U1129: Loss of AMP Communications
DTC U1133: Lost Communication With DVD Player
DTC U1137: Lost Communication With Digital Radio Receiver
DTC U1144: Loss of PHN Communications
DTC U1147: Lost Communication With Onstar
DTC U1151: Lost Communication With Onstar
DTC U1152: Lost Communications With HVAC Control System
DTC U1153: Lost Communication With Climate Control Panel
DTC U1160: Loss of DDM Communications
DTC U1161: Loss of PDM Serial Data
DTC U1162: Lost Communications With Convenience Control System
DTC U1166: Loss of MSM Communications
DTC U1167: Lost Communications With Rear Seat Audio/Rear HVAC
DTC U1173: Lost Communications With Moveable Roof Module
DTC U1176: Loss of RFA Communications
DTC U1192: Lost Communication With Vehicle Theft Deterrent

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Class 2 Serial Data | 1 | U1001-U1254 | 1 | --
Battery Positive Voltage Circuits | -- | U1001-U1254 | -- | --
Ignition Voltage Circuits | -- | U1001-U1254 | -- | --
Module Ground Circuits | -- | U1001-U1254 | U1001-U1254 | --
1. Scan Tool Does Not Communicate with Class 2 Device

### Circuit Description

Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for State of Health monitoring (Node Alive messages). A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. Once an identification number is learned by a module, it will monitor for that modules "Node Alive" message. Each module on the class 2 serial data circuit which is powered and performing functions that require detection of a communications malfunction is required to send a "Node Alive" message every 2 seconds. When no message is detected from a learned identification number for 5 seconds, a DTC U1xxx , where xxx is equal to the 3-digit identification number, is set.

A module with an internal class 2 serial data circuit malfunction or which loses power during the current ignition cycle would have a Lost Communication DTC set by other modules. The modules that can communicate will set a DTC indicating the module that cannot communicate. When no message is detected from a learned identification number for 5 seconds, a DTC U1xxx, where xxx is equal to the 3-digit identification number, is set.

### Conditions for Running the DTC

• | Voltage supplied to the module is in the normal operating voltage range of 9-16 volts.

• | Diagnostic trouble codes U1300, U1301 and U1305 do not have a current status.

• | The vehicle power mode, ignition switch position, requires serial data communication to occur.

### Conditions for Setting the DTC

A message from a learned identification number has not been detected for the past 5 seconds.

### Action Taken When the DTC Sets

The module(s) is never signaled, therefore the specific subsystem(s) will not function.

### Conditions for Clearing the DTC

• | A current DTC clears when the malfunction is no longer present.

• | A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

### Diagnostic Aids

• | Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:

- | A control module on the data communication circuit was disconnected while the communication circuit is awake.

- | Power to one or more modules was interrupted during diagnosis.

- | A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.

- | Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.

- | If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.

• | A control module may have a U code stored in history that does not require any repairs. Issues with late or corrupted messages between control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a control module based only on a history U code.

• | Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue.

• | Communication may be available between the BCM and the scan tool with either the low or high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the low and high speed GMLAN systems.

• | Use to determine if the module uses high or low speed GMLAN serial data communications.

• | Some control modules may not have internal protection for specific control circuits and may open a B+ or ignition fuse. If a fuse is open and the B+ or ignition circuit is not shorted to ground, ensure none of the control circuits are shorted to ground before replacing the control module.

• | This diagnostic can be used for any control module that is not communicating, regardless of the type of serial data circuit it is connected to, providing the vehicle is equipped with the control module.

### Reference Information

#### Schematic Reference

Data Communication Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Data Link Communications Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector of the module that is not communicating.
2. Ignition ON, verify that a test lamp illuminates between all battery positive voltage circuits and ground.

⇒ | If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance. If the circuit fuse is open, also test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the faulty module.

3. Verify that a test lamp illuminates between all ignition voltage circuits and ground.

⇒ | If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance. If the circuit fuse is open, also test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the faulty module.

4. Test for less than 1 ohm between the module ground circuits and ground.

⇒ | If greater than the specified range, test the ground circuit for an open.

5. Test for less than 1 ohm in the class 2 serial data circuit between the non-communicating module and the module that displays the DTC.

⇒ | If greater than the specified range, test the class 2 serial data circuit for open/high resistance.

6. If all circuits test normal, replace the module that is not communicating.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for module replacement, setup, and programming