U0101 GMC Code - Lost Communication With Transmission Control Module
Quick Answer
Modules connected to the high-speed General Motor Local Area Network (GMLAN) serial data circuits monitor for serial data communications during normal vehicle operation. Common causes include Faulty Transmission Control Module (TCM), TCM harness is open or shorted, TCM circuit poor electrical connection. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
- What it means
- U0101 GMC indicates Lost Communication With Transmission Control Module .
- Common causes
- Common causes include Faulty Transmission Control Module (TCM), TCM harness is open or shorted, TCM circuit poor electrical connection.
- Severity
- High - diagnose this code as soon as possible.
- Can I drive?
- Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
Code U0101 GMC Description
The serial data circuit is how the devices in the vehicle communicate with each other. Once the scan tool is connected to the serial data circuit through the Data Link Connector (DLC), the scan tool can be used to monitor each device for diagnostic purposes and to check for Diagnostic Trouble Codes (DTC). When the ignition switch is in RUN, each device communicating on the serial data circuit sends a state of health message to ensure that the device is operating correctly. When a device stops communicating on the serial data circuit, for example, if the device loses power or ground, the state of health message, it normally sends on the serial data circuit disappears. Other devices on the serial data circuit, which expect to receive that state of health message, detect its absence; those devices, in turn, set a DTC associated with the loss of the state of health of the non-communicating device. The DTC is unique to the device which is not communicating, and one or more devices may set the same code. A loss of serial data communications DTC does not represent a failure of the devices that contain the stored code.
What are the Possible Causes of the DTC U0101 GMC?
- Faulty Transmission Control Module (TCM)
- TCM harness is open or shorted
- TCM circuit poor electrical connection
How to Fix the DTC U0101 GMC?
Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.
Code Tech Notes
#PIP3605A: CTC U0101
No engine crank and no communication with TCM
Feb. 29,2008
Models affected:
2006-2007 GMC Sierra Classic
2007-2008 GMC Sierra
Equipped with Allison LCT-1000(RPO-MW7)Automatic Transmissions
Customer may comment that the vehicle will not crank or start/will not shift after the vehicle has been jump-started due to the battery run down or after installation of aftermarket equipment.
Technicians may find no communication with the TCM and or DTC U0101 stored in other modules.
A few TCM's have been returned from the field for analysis because of the above symptoms. The field returned TCM's were analyzed and found to have two battery ground nodes inside the TCM damaged by excessive electrical current passing through the heatsink of the TCM. The nodes affected described as wire bonds are wire tied to pins 9 and 69[battery ground pins] of the TCM.
The TCM is manufactured so that the heatsink of the TCM is isolated from all vehicle electrical systems including vehicle chassis ground. Do not ground welders, battery chargers, electrostatic painters or any other equipment to the heatsink. Disconnect the 80-way connector from the TCM when electrically welding etc. to the vehicle.
To avoid damaging the TCM in this manner, NEVER ALLOW the following:
1. Battery positive wires to touch the heatsink of the TCM [e.g.When jump-starting vehicle]
2. Positive battery power chassis harness wire to pin 10 and pin 70 of the TCM to be wired without fuse protection of 10-15 amps.
3. Physical contact between heatsink and vehicle chassis or other ground circuit points.
4. Electrical grounding of devices such as arc welders, battery chargers, and electrostatic painters, or jumper cables to the TCM heatsink.
Recommendations/Instructions:
Take the following steps to check for damaged ground wire bonds if the symptoms for concern are No Engine Crank and/or No Communications with Tech II.
1. Switch ignition key off.
2. Disconnect the TCM electrical connector and remove TCM from the vehicle.
3. Test for continuity between pins 9 and 69 at the TCM. If continuity exists between pins 9 and 69, then refer to steps in eSI for O Engine Crank and/or No Communications
If the circuit between pins 9 and 69 have no continuity, then the TCM has been damaged/shorted from some sort of external source shorting to the heatsink of the TCM.
1. Inspect for the source that shorted the TCM heatsink.
2. Eliminate or repair the source or condition that caused the damage.
3. Replace damaged TCM after the condition that caused the TCM damage has been corrected.
NOTE: Visual surface pits and carbon deposits are typically found on the TCM heatsink when high energy arcs are generated by shorts between the heatsink and battery power.
Service Bulletin
There is a factory service bulletin for the following GMC models:
2007-2010 GMC Sierra
2007-2010 GMC Yukon Models
2008-2010 GMC Sierra Hybrid
2008-2010 GMC Yukon Hybrid
GMC Factory Service Bulletin OBDII Code U0101
There is a factory service bulletin for some models with U0101 code, the factory recommends that prior to any parts replaced there are several areas that should be inspected:
- One area is the transmission connector pins, for damaged terminals or poor terminal fit.
- Another area is the underhood fuse block for damaged terminals or the poor terminal fit and mispositioned fuses in the fuse block.
- Another area to inspect would be the TCM ground.
- As always powers and ground should always be load checked to make sure the circuits are able to work properly.
- Another common area would be to check the harness for rubbing through on any sharp objects mainly on top of transmission where harness can contact a sharp area on the engine block.
What is the Cost to Diagnose the Code?
Labor: 1.0
To diagnose the U0101 GMC code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.
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Frequently Asked Questions about U0101 GMC Code
1. What does OBDII code U0101 mean for a GMC vehicle?
U0101 indicates that the Transmission Control Module (TCM) has lost communication with other modules in the vehicle, typically due to a network issue.
2. What are common symptoms of code U0101 in a GMC?
Symptoms may include the transmission failing to shift, being stuck in a single gear, reduced engine power, or the check engine light illuminating.
3. What causes code U0101 on a GMC?
Common causes include damaged wiring or connectors, a faulty TCM, issues with the CAN bus network, or a dead battery.
4. Can a bad Transmission Control Module cause code U0101?
Yes, a malfunctioning TCM can lead to U0101 if it fails to communicate with other modules in the vehicle.
5. How do I diagnose code U0101 on a GMC?
Start by inspecting the wiring and connectors for damage, checking the CAN bus network for faults, and testing the TCM for functionality.
6. Can a weak or dead battery trigger code U0101?
Yes, a weak or dead battery can disrupt communication between modules, resulting in the U0101 code.
7. Is it safe to drive a GMC with code U0101?
Driving is not recommended, as the transmission may not function properly, potentially leading to unsafe driving conditions.
8. How can I fix code U0101 on a GMC?
Repairs may include fixing damaged wiring, replacing the TCM, resetting the system, or addressing CAN bus network issues.
9. Does clearing code U0101 solve the problem on a GMC?
Clearing the code may temporarily remove it, but the underlying issue must be resolved to prevent it from returning.
10. How much does it cost to repair code U0101 on a GMC?
Repair costs vary depending on the cause, ranging from $100 to $1,000 for diagnostics, wiring repairs, or TCM replacement.
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