Lost Communication With ECM/PCM "A"
The U0100 code means the vehicle's onboard diagnostic system has detected: Lost Communication With ECM/PCM "A". This is a moderate severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- moderate
- Estimated cost
- $50 - $600 (fuses, grounds, or wiring most often; module replacement only after the circuit tests good)
Network codes mean modules are not communicating. You need a scanner that reads all systems to find the problem.
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Common Symptoms
- Check Engine or related warning light on
- The ECM/PCM "A"'s functions may be degraded or offline
- Often stored alongside other U-codes when a whole bus segment drops
- Symptoms can be intermittent - connector and wiring faults come and go with heat and vibration
Probable Causes (Ranked by Likelihood)
- The ECM/PCM "A" has lost power or ground (check its fuses first) High
- CAN bus wiring damage - chafed harness, corroded connector, water intrusion High
- Failed ECM/PCM "A" no longer transmitting Moderate
- Another faulted module loading down the bus Low
Common Fixes by Vehicle
What techs usually find when diagnosing U0100 on specific vehicles — tap your vehicle for the fix and the exact part:
2007-2018 Chevrolet Silverado/GMC Sierra moderate DIY
Lost communication with ECM/PCM is a common GM code. Check the PCM power supply fuses first (usually 2 fuses in the underhood fuse box). If fuses are good, check the PCM ground connections at the engine block and firewall. A corroded engine ground strap will cause intermittent U0100. If power and grounds are verified, check for CAN bus wiring damage between the PCM and the OBD port.
Labor: 30 min - 2 hours2008-2020 Ford F-150/Escape/Explorer moderate DIY
On Ford vehicles, U0100 often appears alongside a no-start condition. The PCM is not communicating on the CAN bus. Check the PCM fuse first (usually in the battery junction box). If the fuse is good, the problem is often water intrusion at the PCM connector -- Ford PCMs are located in areas susceptible to water damage. Inspect the PCM connector for corrosion. Use FORScan to attempt communication.
Labor: 1-3 hours
2007-2019 Toyota Camry/Corolla/RAV4 moderate DIY
U0100 on Toyota is less common than on domestics and usually indicates a more serious fault. Check battery voltage first -- Toyota CAN bus is very sensitive to low voltage. If the battery is good, inspect the ECM connectors at the firewall. On 2007-2011 Camry, there is a known issue with the ECM connector pins backing out of the housing, causing intermittent communication loss.
Labor: 1-3 hours2008-2020 Chrysler 300/Dodge Charger/Challenger moderate DIY
Chrysler U0100 is frequently caused by the TIPM or a corroded battery cable. The positive battery cable on these vehicles has an integrated fuse block (Intelligent Battery Sensor) that corrodes internally. Check voltage at the PCM fuse with the key on -- if it is below 11.5V, the battery cable needs replacement. Your dealer can confirm whether an open service bulletin or warranty extension applies to your VIN.
Labor: 1-2 hoursFrequently Asked Questions
What does U0100 mean?
U0100 means other modules stopped receiving messages from the ECM/PCM "A" on the vehicle network. It is a communication fault - the module may be fine but unreachable.
Can I keep driving with U0100?
Usually yes for a short period. The affected function may be degraded, and intermittent wiring faults tend to worsen - diagnose soon.
Should I replace the module for U0100?
Not first. Most communication codes are power, ground, fuse, or wiring problems. Replace the module only after those test good - modules are expensive and often require programming.
Mechanics Guide Step-by-step diagnosis
Step-by-Step Diagnostic Procedure
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Scan ALL modules - the pattern of which modules report lost communication localizes the break.
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Check the fuses and grounds feeding the ECM/PCM "A" before suspecting the module itself.
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If many modules are unreachable, test the CAN bus: about 60 ohms across CAN High/Low (two 120-ohm terminators) with the battery disconnected.
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Wiggle-test connectors along the affected harness while watching for the module to drop on and off.
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Only after power, ground, and bus test good should the ECM/PCM "A" be suspected.
Sources 2
This page is built from documented references. Verify against your own service info before repair work.
- SAE J2012Diagnostic Trouble Code Definitions (defines the generic OBD-II code ranges)
- Cross-checked against the SAE J2012:2002 DTC table (2026-08-27)archived scan of the SAE standard — readable copy, not SAE’s current edition
U0100 on Diesel, EV & Equipment
The same code ID appears across other engines and platforms. The diagnostic flow varies by manufacturer — these are the platform-specific breakdowns:
Commonly Affected Vehicles
- All OBD-II vehicles - network (U) codes are standardized across makes
Related Codes
See All Codes & Fixes for Your Vehicle
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