What does it mean?
This is a general communication system diagnostic trouble code that applies to most makes and models of vehicles, including but not limited to Chevrolet, Cadillac, Ford, GMC, Mazda and Nissan. This code means that the transmission control module (TCM) and other control modules on the vehicle are not communicating with each other.
The circuit most commonly used for communication is known as Controller Area Network Bus communication, or more simply put, CAN bus. Without this CAN bus, the control modules cannot communicate and your scan tool may not receive information from the vehicle, depending on which circuit is affected.
Troubleshooting steps may vary depending on the manufacturer, type of communication system, number of wires, and colors of wires in the communication system.
Symptoms
The engine control unit (ECU) uses the signal from the mass air flow sensor to determine the amount of fuel injected. If the sensor does not measure air flow correctly, the fuel injection system will not function properly. Common symptoms include:
- The engine stalls.
- Difficulty starting.
- The engine starts and stalls.
- Reduced power.
- Unstable idle.
Because the MAF sensor readings are used to calculate engine load, the traction control system may stop working and the traction control warning light on the instrument panel may also come on. For the same reason, the automatic transmission's shift pattern may also change.
Diagnostic and repair procedures
A good starting point is always to check the technical service bulletins (TSB) for your specific vehicle. Your problem may be a known problem with a known fix released by the manufacturer and could save you time and money during diagnosis.
First, look for other diagnostic trouble codes. If any of them are related to the bus communication or the battery/ignition, diagnose them first. Misdiagnosis is known to occur if you diagnose the U0101 code before any of the underlying codes have been thoroughly diagnosed and rejected.
If your scan tool can access trouble codes and the only code you get from other modules is U0101, try talking to the TCM. If you can access the codes from the TCM, then the U0101 code is either an intermittent code or a memory code. If you cannot talk to the TCM, then the U0101 code that other modules set is active and the problem already exists.
