83,000 miles. The service light came on tonight. The speedometer showed 90 mph on 2000 Lincoln Navigator

I was only going 15 mph. Shifting seemed not good. Added 5 gallons premium fuel. Ran better. Speedometer acted normal. rpm jumped at times. light still on. running rough. made it home. Where do I start to fix this?

Asked by for the 2000 Lincoln Navigator
have it scan tested for codes and go from there
1 more answer
start by pulling code(s)