I take it back
Yesterday I was all set to figure out my "service engine soon" issue on my own. Well, I DID check the gas cap; it was fine. I also checked all the fluids in the car and made sure nothing was leaking. All that was fine too. What I DIDN'T do was drive out to an auto parts store to get a diagnostic test. Honestly, I reasoned that even if I did, I probably wasn't going to be changing any O2 sensors or air filters myself. I decided this morning to just take it to the car repair place I trust the most. They're within walking distance of work and really, at some point, I have to just trust someone to fix my car. I can't do it. In the end, the cost was really not much more than a car payment and ONE car payment is a lot less painful than financing $15,000 or something for a "newer" car.
So while I might not have done all I could to pinch pennies in this situation, I did what I knew would work out best for me. After all, car repairs are why I have a savings account.