P3478 indicates that the powertrain control module has detected a performance problem in the exhaust valve control circuit for cylinder 10. This typically means the valve is not opening or closing as commanded, affecting the variable valve timing system.
The actuator that moves the exhaust valve can wear out or seize, preventing proper operation.
Open, shorted, or high‑resistance wires can disrupt the signal from the control module.
If the sensor that reports valve position is out of range, the module may record a performance fault.
A bad connector can cause intermittent communication and trigger the code.
Only after ruling out mechanical and wiring issues should the PCM be considered, and even then replacement is rarely needed.
No. The PCM is rarely the cause of this code; replacing it without verifying the valve, wiring, and sensor will likely not fix the problem and can be costly.
Occasionally a temporary condition can clear the code, but if the underlying fault remains, the code will return on the next drive cycle.
P3478 is specific to the exhaust valve control circuit and does not directly indicate a catalytic converter problem, though poor valve operation can affect emissions.
Because it affects valve timing on one cylinder, the engine may run rough and lose efficiency. It should be addressed promptly to avoid further wear or damage.