DTC U1001-U1254. Chevrolet Tahoe

For 1990-2009 cars only

Makes 🢒 Chevrolet 🢒 2001 🢒 Tahoe - 4WD 🢒 Body and Accessories 🢒 Data Link Communications 🢒 DTC U1001-U1254

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. 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 is set.

The Control Module ID Number list provides a method for determining which module is not communicating. 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. Use the Control Module ID Number list in order to determine which module is not communicating and the Lost Communications with XXX diagnostic table in order to diagnose the malfunction. When no message is detected from a learned identification number for 5 seconds, a DTC U1xxx is set.

Control Module	ID Number
PCM	016
TCM	024
TCSCM	026
EBCM	041
SCM	056
BCM	064
SDM	088
IPC	096
DIC	097
ONS	151
HCM	152
MSM	166
MMM	169

When more than one Loss of Communication DTC is set in a single module or among multiple modules, diagnose the DTCs in the following order:

Current DTCs before history DTCs except as specified in the diagnostic tables.
The DTC which is reported the most times.
From the lowest number DTC to the highest number DTC.

Conditions for Running the DTC

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

•	DTCs U1300, U1301 or U1305 do not have a current status.

•	The vehicle power mode 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.

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, without a repeat of the malfunction.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

A module which loses power during an ignition cycle will cause other module to set lost communication DTCs.

The malfunction is due to an open in the class 2 serial data circuit or an open in the module.

The module which was not communicating due to an open in the class 2 serial data circuit may have set lost communication DTCs for those modules that it was monitoring.

The modules which can communicate indicate the module which cannot communicate. You must clear the DTCs from these modules to avoid future misdiagnosis.

Step

Action

Yes

Test the following circuits of the module that is not communicating:

•	Switched battery input voltage

•	Ignition input voltage

•	Ignition output voltage

for an

•

Open

•	Short to ground

Refer to:

•	Control Module References in Body Control System for the applicable schematic

•	Circuit Testing in Wiring Systems

•	Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 9

Go to Step 2

Test the ground circuits of the module that is not communicating for an open.

Refer to:

•	Control Module References in Body Control System for the applicable schematic

•	Circuit Testing in Wiring Systems

•	Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 9

Go to Step 3

Inspect for poor connections at the star connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 7

Go to Step 4

Test the class 2 circuit of the module that is not communicating for an open between the module and the star connector. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 7

Go to Step 5

Inspect for poor connections at the following locations, for the module that is not communicating:

•	Switched battery input voltage circuits

•	Ignition input voltage circuits

•	Ignition output voltage circuits

•	Ground circuits

•	Class 2 circuits

Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 7

Go to Step 6

Important: Replace and reprogram the module which is not communicating. Refer to Control Module References in Body Control System for the appropriate repair instructions.

Did you complete the replacement?

Go to Step 9

Connect a scan tool.
Turn ON the ignition leaving the engine OFF.
Select the Display DTCs function for the module which was not communicating.

Does the scan tool display any DTCs which do not begin with a "U"?

Go to Control Module References in Body Control System for the applicable Diagnostic System Check

Go to Step 8

Use the scan tool to clear the DTCs.

Did you clear all DTCs?

Go to Step 9

Select the Display DTCs function for the modules which had the Lost Communications with XXX DTC set.

Does the scan tool display any DTCs which do not begin with a "U"?

Go to Control Module References in Body Control System for the applicable Diagnostic System Check

Go to Step 10

Use the scan tool to clear the DTCs.
Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared.

Did you complete the action?

System OK