Though there are several additional facts necessary to fully understand your potential claims, it looks like you might have claims under your state's lemon laws (assuming there's a warranty on the car) and/or potential claims against the seller for misrepresenting the true condition of the car. Also, it appears there might be a negligent repair claim against PepBoys, since from your description they were the last ones to replace the car's oil. In any event, find a consumer law attorney near you and provide him or her with a thorough depiction of what happened. Try www.naca.net if you have trouble locating a consumer law attorney in your area. Good luck.
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Sounds like you have a cause for a Small Claim Court case. Take any documents pertinent to the sale and anything else you have that documents the problems and ask the Judge to order a full refund. Good Luck!
The warning light for me IS the warning light. Dealer knew or should have known something was up which may bring your state's UDAP statute into play. Definitely contact a consumer protection atty right away. Try http://www.naca.net for someone competent in lemon law and auto fraud matters.
Washington's "Lemon Law" (RCW 19.118) applies to new vehicles, and to manufacturers rather than dealers. That said, you probably have a claim against the dealer for misrepresntation and/or breach of warranty. Small Claims might be the right avenue for you -- In King County, the dollar limit is $5,000. You can handle a small claims case yourself. If the amount in controversy is higher, you may need to retain and attorney.