Should I drain the tank ??
 

I just bought a 2003 Jag xk8 coupe went to the gas station and mindlessly filled it 3/4 up with regular when I got home and restarted it the engine light came on could this have caused it and if so should I drainthe tank Thanks pattie

The low octane gas had nothing to do with the engine light coming on. Have someone with an OBDII reader find out what codes are being generated.

Thank you for your reply :)

What likely happened is they cleared the codes before you bought the car and now, whatever the problem is, is showing up again. Without knowing the code you can't determine what the real problem is. There is an impressive list of things that can fire that light off. Odds are it is part of your smog system.

Pattie,

You could do a lot worse than getting a cheap ELM327 scanner from eBay or Amazon and the corresponding phone app. It will read all standard codes and can a long way towards getting to a diagnostic. Most parts store offer the scan service for free, too.

As far as draining the tank goes, I don't think that's necessary. Burning a single tank of low-octane gas shouldn't do any harm. If you're concerned, you can always add of bottle of octane boost from AutoZone or Walmart. Otherwise just wait until the tank gets lower and top it off with high-test a few times, eventually replacing all the current gas with high-test. (P.S. I - and many others - use mid-range gas with no ill effects. I've been doing so since 2011 and my engine is smooth and powerful, no pings, no knocks.)