I have an 89 IROC with an 86 305 TPI and a mild cam. I have recently hooked my car up to my dads diagnostic computer to check my codes, I was getting a code 13 and a 42. I adjusted the TPS and cleaned my incredibly dirty IAC, the haynes manual said those could cause a 42. I cleared the codes and drove it for a while ses came on, jumped the system and only got a 13, we then hooked it up to another diagnostic and the 42 came back along with the 13, I changed my o2 and cleared the codes, drove it for a while and the ses came on, jumped it and got nothing but a 13. I have no AIR system, could this be causing a 13? Also I was told the 42 is my ignition module, I thought my car wouldnt run if that was bad? And why does the 42 only show up on the diagnostic computer? Any help is much appreciated. -James

42 does point to the ignition module. However it seems that other things can confuse the computer and throw that code. I had that happen with mine when I had a shorted out injector. No check engine light, just a code when I scanned it. Fixed the injector, no more code 42.

As for your code 13, aren't there 2 O2 sensors on your car? My '91 with a 305 has two (I think. Second guessing myself the more I think about it ). Double check that you don't have two. If you do replace the other one. Also check the connectors to be sure that they are tight and not soaked with oil etc etc. Does your Dad's scanner also do op-data? If it does check the reading on the O2 sensor. It should stay pretty steady.

Nope, only one o2, its an upstream. Connectors are good and clean, I dont know if it does op-data. I also forgot to mention that the car isnt going into closed loop, and its running like crap. Big hesitation when you hammer it, and the top end is very much lacking, another issue is no matter what the car shifts at 4400 rpm under WOT. Very odd.