Fix U0100: Mechanic’s solutions that WORKED

The code U0100: Lost Communication with Engine Control Module (ECM) / Powertrain Control Module (PCM) “A” indicates a communication breakdown between the vehicle’s Engine Control Module (ECM) or Powertrain Control Module (PCM) and other parts of the vehicle. This fault can stem from various issues such as electrical problems, a faulty ECM/PCM, or wiring issues within the communication network of the vehicle.

U0100 quick overview

Meaning	U0100: Lost Communication with Engine Control Module (ECM) / Powertrain Control Module (PCM) “A”
Is it serious?	Potentially serious. Can affect vehicle’s drivability and functionality.
Possible causes	– Faulty ECM/PCM – ECM/PCM harness is open or shorted – Poor electrical connection in ECM/PCM circuit – Can Bus off
How to diagnose?	Check battery: Ensure it’s in good condition and fully charged. Inspect connections: Look for loose or corroded battery terminals. Scan for other codes: Other codes can help pinpoint the issue. Monitor CAN bus: Use a scope to check CAN bus signals. Test circuits: Perform voltage and resistance checks on relevant circuits. Update software: If a TSB is available, consider ECU reflash. Inspect grounds: Verify good ground connections for modules.
What is Canbus?	Canbus system is a connection between your control modules to allow communication between them.

How to fix U0100 – learn from mechanics

I looked into U0100 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Buick

Vehicle information 2010 Buick LaCrosse CXL 3.0L

Issue + repair history

• Check Engine Light on
• Traction Control Light on
• Drivability issues (transmission defaults to certain gears due to U0100 code)
• No previous repairs
• Codes: P0700-00, U0073-00, U0100-00

Other OBD2 codes + test results

• No additional codes or test results mentioned

What are people suggesting to fix issue

• Check Transmission Control Module for Diagnostic Trouble Codes
• Address U codes on the bus network
• Check powers and grounds at PCM and TCM
• Wiggle harness to see if fault code reappears
• Use a lab scope to check CAN bus signal
• Disconnect modules one at a time to identify issues
• Check ABS actuator and wiring harness
• Investigate CAN bus for shorts, high resistance, or open circuits
• Consider the ECM or wiring to be the potential cause

What was the final fix

• Replaced the Engine Control Module (ECM) and programmed it to address the condition.

Case #2: Cadillac

Vehicle Information
2010 Cadillac STS, RWD, 3.6L V6

Issue + Repair History
Reduced power mode, codes ECM P0700, P2544-00, TCM U0100, EBCM U0100-00, U0146-00. Previous misfire codes P0300, P0301, P0306, and P0008. Replaced spark plugs, timing chain, VVT actuators, and induction service.

What are people suggesting to fix issue
Scope data lines and wiggle tests Unplug EBCM and test drive Load test ECM power and ground Check wires, connections, and grounds Scope communication lines during the event Check high-speed circuit resistance at DLC and follow communication flow charts

What was the final fix
Replaced radio/navigation unit, laid new twisted pair wire after discovering damaged high-speed communication wire during wiggle test. Issues resolved after repair.

Case #3: Mazda

Vehicle Information
2003 Mazda 6 S 3.0L

Issue + Repair History
Initial issue: MIL Lamp on. After a quick acceleration, code P0303 (cylinder 3 misfire) triggered but misfire wasn’t felt. Concurrently, A/C wasn’t cooling with low pressures, system cycling rapidly. Later, codes U0100 (communication error with PCM) and U0121 appeared, with harsh transmission shifts, and AT and TCS lights on. After restarting, AT and TCS lights off, but MIL remained. Vehicle bought at auction, likely due to these issues.

What are people suggesting to fix issue

• Procedure for isolating the U-code from Mazda tech site
• Check grounds and possibly PCM, inspect main harness
• Replace bad ignition coils and spark plug, particularly the ones under the intake as they tend to overheat and break down

What was the final fix
Dealership reprogrammed PCM and TCM, which resolved U0100 and U0121 codes. The fix was confirmed effective after 150+ miles without the MIL Lamp turning back on.

Case #4: Jeep

Vehicle Information
2014 Jeep Grand Cherokee Limited 3.6L

Issue + Repair History
Came in with check engine light and an instance of not cranking, requiring a jump start. Found multiple CAN failure codes: U0100, U0121, U0102, U0103, U0212, U0140. Discovered a loose negative terminal and a bad battery, both replaced. Codes cleared but reoccurred intermittently. BCM was flashed following a TSB for communication issues. Unable to reproduce the issue consistently for further diagnosis.

What are people suggesting to fix issue

• Check B+ connections between battery and fuse box for voltage drop
• Ensure PCM connectors are fully seated
• Investigate potential bad earth in the information module in the roof
• Disconnect modules with communication codes and check wires for resistance
• Consider temperature effects on ECM, possibly going open when warm
• Use a scope to check CAN signal disruptions

What was the final fix
Upon inspection with a Fluke scope and meter, CAN signals appeared normal. Disconnected CAN lines for TCM, SCM, and ABS, and checked each for resistance and signal, all tested good. Discovered and replaced a broken ground strap near the alternator. Suspected a contact issue at the TCM. After several test drives with no CEL, the vehicle was released.

Case #5: Land Rover

Vehicle Information
2008 Land Rover LR2 SE 3.2L

Issue + Repair History
Customer reports that RPM flares above normal when approaching and going over a speed bump. Unable to duplicate the problem during test drive. Only potentially related TCM code found: P0725 – Engine speed input circuit. Numerous other codes found across various modules, most notably U0100 in multiple modules. Checked transmission fluid level, which was okay.

Advice from Other Technicians

• Don’t be concerned with “B” or “U” codes as they may result from low battery power. P0725 could be due to the speed sensor inside the transmission. Suggested bringing it to the dealer for a software update.
• Check if four-wheel drive is working properly. A problem with VCU could cause drive loss when a wheel comes off the ground.
• Check transmission fluid level when cold, as going over a bump could cause the pump to suck air and cause engine flare.

What was the final conclusion
Diagnostic procedure for P0725 according to Land Rover involves checking for communication faults between modules. Suspected issues might be related to a low battery condition as the vehicle had been parked for a long time. Additionally, a resetting P0175 code is present, for which a bulletin suggests a reflash. Advised the customer to visit the dealer in Mexico for this service.

Common Advice	Brief Description of Diagnostic Step
Check battery and connections	Ensure the battery is in good condition and all connections are secure, as low battery power can cause various communication errors (U codes).
Update software/ECU flash	If available, updating the software or performing an ECU flash can resolve issues related to the fault codes, especially if there’s a TSB.
Check communication between modules	Use diagnostic tools to ensure proper communication between the various control modules; interrupted communication can cause these fault codes.
Inspect and test relevant wiring and connectors	Examine wiring harnesses and connectors for signs of damage, corrosion, or loose connections that might lead to communication failures.
Check module grounds and power supply	Verify that all modules have good grounding and proper power supply, as poor grounding can cause communication issues.
Monitor and scope CAN bus signals	Use an oscilloscope to monitor CAN bus signals for any disruptions or irregularities that could indicate communication faults.
Test resistance and voltage in communication circuits	Perform resistance and voltage drop tests on communication circuits to identify any circuit issues that might lead to communication faults.
Consider temperature effects on components	Some components might fail intermittently due to temperature changes; consider if issues worsen when components are hot or cold.
Check for mechanical issues related to the fault codes	In some cases, mechanical issues like a slipping transmission could cause fault codes that seem to be related to communication failures.
Address any known bulletins or recalls	If there are any manufacturer bulletins or recalls related to the fault codes, ensure these are addressed as they can provide known fixes.