My check engine light came on and ive noticed alot of power lost when i floor it from 80 plus.
Can a failed o2 sensor be the problem? any other opinions?and yes gas mileage went down a little. No loose caps.

 

Some issues to take a look at like what kind of spark plugs, wires, cap, rotor, o2 sensors do you have. Also how many miles do you have on these parts? How many miles do you have on the engine? What size engine? Do you use synthetic or conventional? These are some stater questions to help us figure what is going on? Take it by an autozone and have them print out the codes so you can have them on hand.

 

Right.

We need more info and the code that you tripped. But yes to answer your question.

 

125000 km champion plugs changed about 10 000 kms ago. the rest is all original. also when i changed my plugs they were black. running rich? 5.9 l 360. synthetic oil.

 

did you change the wires, cap, and rotor also? If not those can be the problem. Do you have any spark plug wires laying on the engine?

 

Possible rich burn. I think you aren't getting good spark.

Like DRO318 says, do a ful tune up on that beast.

Regular Champion plugs gapped .040 (these are the $1.50 regular plugs)
OEM or MSD 8.5mm wires.
Cap and Rotor (get the upgraged Brass Contacts)
Clean the TB and sensors.
O2 Sensors might be going out.

 

ran the code p0442 Evaporative Emission Control System Leak Detected (small leak). checked gas cap. code still there. any idea how much this could cost?

 

Do a search on the code in the forum. There are some other ideas and cost comparison.