U1821
DTC U1821
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptor
DTC U1821 00
- Lost Communication with 14V Power Module on Powertrain Expansion Communication Bus
Diagnostic Fault Information
Circuit/System Description
Control modules connected to the serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the control modules. The control modules have prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. Each message contains the identification number of the transmitter module.
Conditions for Running the DTC
* The system voltage is between 9-16 V.
* The vehicle power mode master requires serial data communication to occur from this specific module.
Conditions for Setting the DTC
A supervised periodic message that includes the transmitter module availability has not been received.
Action Taken When the DTC Sets
The control module is never signaled. Therefore, the specific subsystems 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 control 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 control module and the scan tool with one or more GMLAN serial data systems inoperative. This condition is due to the control module using multiple GMLAN busses.
* Use Data Link References Initial Inspection and Diagnostic Overview to determine what GMLAN serial data communications the non communicating control module uses.
* The control module 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 non communicating control module.
Reference Information
Schematic Reference
* Data Communication Schematics [1][2]Electrical Diagrams
* Control Module References Programming and Relearning
Connector End View Reference
Component Connector End Views Hybrid/EV Battery Pack X1
Description and Operation
Data Link Communications Description and Operation Description and Operation
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
Scan Tool Reference
Control Module References Programming and Relearning for scan tool information
Circuit/System Verification
1. Verify that DTC U0074, P06E4, B1428, B1370, B1380, B1440, B1441, B1325, or P0562 is not set.
• If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle Diagnostic Trouble Code (DTC) List - Vehicle.
2. Verify that DTCs U1817, U1818, and U182D are not set.
• If all the DTCs are set, refer to DTC U0074 U0074.
Circuit/System Testing
Note: Use the schematic to identify the following:
* The control module location on the powertrain high speed GMLAN serial data circuits
* The control module B+, ignition, ground, accessory wake up serial data 2 and serial data circuit terminals
1. Vehicle OFF, disconnect the low voltage harness connector at the K1 14V power module.
2. Test for less than 5 ohm between the K1 14V power module case and ground.
• If greater than the specified range, test the ground strap for an open/high resistance.
3. If equipped, vehicle in Service Mode, verify that a test lamp illuminates between each B+ circuit terminal and ground.
• If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the K1 14V power module for a short to ground. If the circuits test normal, replace the K1 14V power module.
4. If equipped, vehicle in Service Mode, verify that a test lamp illuminates between each ignition circuit terminal and ground.
• If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the K1 14V power module for a short to ground. If the circuits test normal, replace the K1 14V power module.
5. If equipped, vehicle in Service Mode, verify that a test lamp illuminates between the accessory wake up serial data 2 circuit terminal and ground.
• If the test lamp does not illuminate, test the entire circuit, including the B+ circuit to the K114B hybrid powertrain control module 2, for an open/high resistance or short to ground. If the circuits test normal, replace the K114B hybrid powertrain control module 2.
6. Vehicle OFF, disconnect the harness connector at the control module setting the DTC.
7. Vehicle OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 2 ohm in each powertrain high speed GMLAN serial data circuit between the K1 14V power module and the control module setting the DTC.
• If greater than the specified range, test the serial data circuit for an open/high resistance between the K1 14V power module and the control module setting the DTC.
8. If all circuits test normal, replace the K1 14V power module.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
* GMLAN Wiring Repairs GMLAN Wiring Repairs
* Control Module References Programming and Relearning for control module replacement, programming and setup