i have an '01 4.2 V6 and have had a "service engine soon" light since i got the truck finally hooked it up to a scanner and had three codes (p401, p171, p174) which 401 is the EGR and the 171 and 174 are the 2 banks running lean, i have since replaced the EGR, cleaned the EGR to intake tube and replaced the feedback sensor and it ran fine for a while then started giving a p303 cylinder three misfire which happened when a boot came off the coil pack we checked the plug which was black but managed to clean it and their was no other visible damage so we reused the plug and replaced the boot on the coil pack and the truck ran fine for a couple of months and the code went away on its own even though the "service engine soon" light stayed on and periodically flashed when the truck tried clearing the problem itself. that was until tonight when i was driving home from work and was acelerating out of a turn and the truck made a noise like a low voltage stun gun which it did 3 times and only while making a right turn so i ran the scanner again and all 4 codes have come back and i cannot figure out what is wrong with it. im going to replace the plug and someone has suggested it may be a crack in the coil pack

any ideas?

 

You are going round and round with this because you are addressing with guessing.
What is a 401 code and how does it's test work?
What is the 171/174 codes and what can cause them?
The 303 code is a cylinder ignition failure. What is causing it?
You need to work at a deeper level and understand or these will keep coming back.
401 code is a low flow test failure. It is a test set up by a program routine on a skeduled basis. The vacuum EVR regulator is involved, the EGR and the DPFE with it's connecting hardware. Lots of places the fault can be. The PCM must see two test failures in a row to call it a failure.
The 171/174 faults mostly is an intake air leak being detected as a lean condition sending the fuel tables out of spec (on the rich side) and causing the code for both banks. It's not the Ox sensor.
The 303 cylinder is still in fault and can be the coil pak.
You own a computer controlled system and works much more involved, if you are going to do the work yourself.
Codes don't tell you what parts are causing the issues but only the system in trouble, then you need to address the whole system for the cause.
Otherwise luck is your friend if it comes by.
Good luck.

 

My 171/174 code ended up being a slight leak in the intake gasket. 4 years and $300, along with a diagnosis from a really great mechanic and the problem is solved.

 

Search those codes on this forum. You probably have a vacuum leak some where. How many miles are on your truck? If it's in the upwards of 80K you probably need a new coil pack.