Q: Performance Restricted Warning and Check Engine Light - 2004 Jaguar XK8

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I have a well-maintained car with 167,000 miles on it. It's been running great. A while ago, while I was on a trip, I had to fill in the gas tank with a middle grade gas instead of Premium I've been using. The next day, the warning came on indicating the restricted performance. After a while during driving, the warning disappeared. I filled in Premium after that several times. I took the car to a shop where they analyzed it and had the computer reset. However, the check engine light came on illuminated in yellow. It was followed by the same restricted performance warning. These disappeared when I filled the car with premium again. Why did this happen? Is there anything I should worry about?

Hello. The engine knock sensor is retarding the ignition timing to match what the lower octane fuel can support in terms of “timing advance.” There is nothing to worry about. The engine computer is just operating the engine so as to minimize detonation or pre-ignition (aka “spark knock”). Given your description though, you should always use premium fuel. If the light continues to come on or you begin to notice drivability concerns, I recommend having a professional mechanic, like one from YourMechanic, diagnose the Check Engine Lights and warning lights in order to have this corrected.

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