U-Codes and Network Faults

What does U0100 lost communication with ECM mean?

U0100 means another module stopped hearing from the engine computer on the network; the cause is usually power, ground or wiring to the ECM, or a network fault, rather than a failed module.

Why do I see U-codes in a lot of modules at once?

Short answer: Multiple U-codes across modules usually mean a shared cause: low battery voltage, a bad ground, a network wiring fault, or a module that drags the bus down.

When a battery drops low during cranking or sits discharged, modules brown out at different times and log lost-communication codes against each other. That alone can fill a full-system scan with U-codes. Test and charge the battery first, then clear codes and see which return.

If they come back, look at the pattern. Codes all pointing to one module suggest that module lost power, ground or its network connection. Codes spread across many modules on the same bus suggest a bus problem, such as a shorted wire, water in a connector or a failing module that pulls the network down.

A practical test is to unplug modules on the affected bus one at a time while watching network voltage or scope patterns. When the bus recovers, you found the troublemaker. Check service information for the network layout, since many vehicles run several separate buses joined by a gateway.

What does U0073 control module communication bus off mean?

Short answer: U0073 means a module detected that a communication network went off or stopped working, often due to a short on the bus wires, a failed module, a wiring issue or a terminating resistor problem.

Modules share data on networks such as CAN. When the bus fails, modules cannot communicate and many may set U-codes. U0073 is defined as communication bus A off, though specifics vary by maker.

Diagnosis starts with a full system scan to see which modules respond. With the battery disconnected as service information directs, measuring resistance across the CAN high and low wires at the diagnostic connector checks the terminating resistors. A scope shows whether the bus signals look correct.

Disconnecting modules one at a time while watching the bus can find a module pulling it down. Check connectors for water entry and wiring that has been pinched or chafed. Disconnect the battery before unplugging modules as directed, and follow airbag module precautions.

What does U0121 lost communication with ABS module mean?

Short answer: U0121 means a module stopped receiving expected messages from the ABS module, which can be caused by a power or ground fault at the ABS module, a wiring issue, or a failed ABS module.

Other modules, such as the engine and instrument cluster, rely on data from the ABS module, like wheel speeds. When messages stop, they set U0121.

Check whether the ABS module communicates with a scan tool. If it does not, check its fuses, power and ground circuits under load, and connector condition, since ABS modules sit in harsh locations near wheels and road spray. Then check the network wiring to the module.

If it communicates but codes keep setting, look for intermittent connection problems. Low battery voltage can also cause communication codes. Test brakes after any repair, and follow ABS bleeding procedures when the hydraulic unit is opened. Support the vehicle safely on a lift or rated jack stands if work needs it raised.

How do I find which module is pulling down the CAN bus?

Short answer: Watch the bus on a scope or monitor resistance and voltage while you disconnect modules one at a time; when the bus recovers, the last module or its wiring is the likely cause.

A shorted module or wiring fault can disrupt the whole network. Start with a scan to see which modules respond. Use the wiring diagram to see how modules connect and where the access points are.

Connect a scope to CAN high and low at the diagnostic connector. With the bus faulted, disconnect modules one at a time, as service information permits, and watch for the signal to return to normal. If unplugging modules does not help, check wiring branches between modules for shorts to power, ground, or each other.

Some modules need special handling, such as airbag modules. Disconnect the battery when service information says to, wait the specified time for airbag systems, and follow the maker's procedures.