2000 Lincoln Navigator Q&A

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

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? -
Answer 1
have it scan tested for codes and go from there -
Answer 2
start by pulling code(s) -