A C0278 error code is a diagnostic trouble code (DTC) for Secondary Air Injection System Relay Circuit Malfunction. This code is set when the powertrain control module (PCM) detects a problem with the secondary air injection system relay circuit.
The most common symptom of a C0278 error code is that the check engine light will be illuminated on the dash. Other symptoms may include:
-Rough idle
-Reduced fuel economy
-Poor engine performance