U0066 indicates that the engine or network diagnostic system has detected an open circuit on a manufacturer-defined serial data line called Vehicle Communication Bus E. An 'open' means the bus circuit has lost continuity somewhere between modules, so one or more control modules cannot send or receive messages. This is almost always a wiring, connector, or termination problem rather than the engine computer itself.
The data line may be pinched, rubbed through, corroded, or broken, creating an open or short to ground/power that stops communication.
Poor contact at a module connector, splice, or inline connector interrupts the bus signal and sets the open-circuit code.
Many serial buses require matched terminating resistors; a missing or out-of-spec resistor can cause an open-bus fault or erratic communication.
An internal fault in a module attached to Bus E can pull the data lines high or low, making the rest of the bus appear open.
This is uncommon; the engine computer is only a realistic cause after the wiring, connectors, termination, and other Bus E modules have been ruled out.
No. U0066 is most commonly caused by wiring, connector, or termination resistor problems on Bus E. PCM/ECM replacement is rarely the correct fix and should only be considered after the bus and all other modules are proven good.
The vehicle may still run if Bus E does not control critical systems, but affected modules may be offline and related functions may not work. It should be diagnosed promptly to avoid further issues.
An open Bus E circuit breaks the data path, so modules on that bus cannot be reached by the scan tool or by other modules on the network.
A scan tool that can read network codes, the vehicle wiring diagram, and a digital multimeter are usually required; an oscilloscope can help if intermittent signal faults are suspected.