Service Engine Soon

I hit a deer ... going 70mph ... poor bambi ... poor truck. I replaced my radiator. I will be replacing other front end parts as I can. Now my service engine soon light comes on. My truck is driving fine. Is there anyway to determine engine trouble codes without buying diagnostic tools and is there a free list of what the codes are? Thanks

Some auto parts stores will read the codes for you.

Thanks .. AutoZone did it for free .. got codes P0171 and P0174. According to AutoZone, the air fuel ratio was lean and the air sensor was probably the culprit (one code by itself would have indicated the fuel regulator). I had to take off the air manifold when I replaced the radiator. After I put everything back together, the the idle was rough the first time I started it. It has started and ran good since then. The air sensor, next to the air filter, has no apparent external damage. It's been 1 week and I'm going to leave it alone for the time being. The owners manual says it is an emmission problem. Can this lead to serious engine damage? Thanks