U0100 on Toyota: How to Diagnose & Fix
Lost Communication with ECM/PCM
1 Toyota model · 1 documented issue · 1 critical
U0100 on Toyota by model: symptoms, diagnosis and fix
Toyota recalled roughly 1.13 million 2005-2008 Corolla and Corolla Matrix vehicles built with the 1ZZ-FE engine and two-wheel drive because the engine control module was improperly manufactured. Cracks can develop at solder points or on varistors on the ECM circuit board. Symptoms escalate from an illuminated check engine light to abnormally harsh automatic transmission shifting, a no-start condition, and in the worst case the engine shutting off while the vehicle is being driven. Toyota's internal campaign designation is A0J. Owners of 2004 cars report the same failure pattern with a mechanically similar module even though that year sits outside the recall population, so a 2004 with these symptoms should still have the ECM considered. Note the recall is limited to 2WD 1ZZ-FE cars - AWD and XRS models were built with different modules.
Common Symptoms
- Engine stalls without warning while driving
- Engine cranks but will not start
- Harsh or abrupt automatic transmission shifts
- Check engine light on with no obvious mechanical cause
- Intermittent loss of communication with the scan tool
How to Fix
Free remedy under NHTSA recall 10V384000 / Toyota A0J: dealers inspect and reprogram or replace the ECM at no charge. Check the VIN on the NHTSA or Toyota recall lookup first, because many of these cars were remedied years ago. Outside the recall population the fix is ECM replacement or board-level repair (a reflow/reman of the original module is generally far cheaper than a new unit and keeps the immobilizer pairing intact). Do not chase a random no-start with parts before ruling the ECM in or out.
Sources for this issue (3)
U0100 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does U0100 mean on a Toyota?
U0100 stands for "Lost Communication with ECM/PCM." U0100 is a network communication code meaning a module has lost communication with the engine control module / powertrain control module (ECM/PCM) over the vehicle's data bus. The ECM/PCM is the brain that runs the engine, so when other modules can no longer 'talk' to it, the effects can be serious — a no-start, stalling, multiple warning lights, and many other modules logging faults at once. This is almost always a wiring, connector, power/ground, or bus fault, or a failure of the ECM/PCM itself, rather than a single sensor. It should be diagnosed promptly because the vehicle may be undrivable. On Toyota it is documented across 1 model: Matrix.
What are the most common causes of U0100?
The most common causes of U0100 are: Loss of power or ground to the ECM/PCM, CAN bus wiring damage, short, or open circuit, Corroded, loose, or damaged connectors at the module, Blown fuse feeding the ECM/PCM, Failed ECM/PCM, Another faulty module pulling down the data bus, Water intrusion or corrosion in a connector/harness. The specific cause varies by vehicle.
How much does it cost to fix U0100 on a Toyota?
Repair costs vary widely with the root cause and Toyota model; each issue on this page carries its own estimate where one is published.
Looking for U0100 on a different make?
View U0100 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.