94 Silverado- 136K miles - intermittent "Service Engine Soon" light with code 78 - Exhaust Valve Control Solenoid Circuit Bank The truck seems to be running fine - no unusual noises or abnormal instrument readings - 16.1 mpg driving around town. The warning light is very intermittent - never stays on more than 5-10 seconds. Does anyone know: 1) What's the probable cause/corrective action? 2) Does it need to be fixed right away or is it simply an emissions or other no-serious issue that can wait-I'm getting ready to take a 1200 mile road trip.