Well Im new here. I have a 1994 F150 5.0 auto. I've had it for about a year. Ive searched all over for this problem and can't find anything that helps me out. I've been having a problem with the truck idling low then jumping up. It will do this mainly when stopped. I used the jumper wire for the diagnostic test. I got 121 twice and 542 twice. Nothing for the continuous codes. Not sure what I did wrong here. I tried to look into the 542 code pcm to ground, but can't figure where to start. Any help would greatly appreciated.

 

Start with the pmc to make sure the bolt on the pmc is not loose. Than there is about 5-7 ground wires though out the truck. Clean all ground wires. 121 could be the tps or bad ground also. Google your pmc,year,model and engine size. It has 3-5 pins in it that go to ground.

 

Thanks. I'll check it out