check engine light, after tune-up help.

my friend and i did a tune-up on his 00 xj this weekend. he hadnt done a spark plug change since he owned it, well probably the whole life of the jeep- 107,000. so we changed the autolites with mopar plugs this weekend, we had them all snugged up put the rail back on and took it for a spin. he said he noticed it shaking alittle when stopped and vibing. so i went back and checked all the plugs pulled each one and re-insterted it making sure i didnt leave one untigtened. we get to the last plug, at the very front of the block and it wont tighten up, it gets to a certain point and then unloosens. we snug it up and leave it as is. still vibed alittle, but i figured it was new plugs. he emails me today saying he had a flashing check engine light, and then a solid check engine light- still vibing, runs strong though. what could it be? hwo can he check the code? we also changed the oil. i changed the plugs on my tj and had no problems. bad plug? what could it be.

sounds like a stripped head the plug has to tighten all the way or it will leak compression the flashing check engine light means its missfireing i dont know how to check the codes without a scanner but if you look around the site im sure you will find that answer the fix would be to heli coil the stripped hole but if your not experienced with this type of repair i strongly suggest you take it to a shop