I changed the spark plugs in my '05 5.4 at about 72k miles. Prepped it with a bottle of Techron to the tank when 1/2 full 3 times. Used some Deep Creep on the plugs engine warm after blowing out debris in the spark plug wells and let it sit overnight. Got lucky and got all plugs changed with no breakage. Used the Motorcraft SP515 plugs with the nickel antiseize on the shrouds above the electrodes. With a fuel filter and air filter change at same time, truck ran great for a while.

After about 1000 mi on the new plugs, I noticed that the idle quality had declined, the engine didn't seem as peppy as it was right after the plug change and the fuel mileage dropped from around 16 to 13/14's.

So after some online research here at the forum and other places, decided to change the upstream O2's and see if that would help. I had run the Torque Pro app on the OBDII and it showed no misfires or any other errors or codes.

I got a couple Motorcraft sensors and an offset 22mm(7/8) sensor socket from Amazon. Warmed the engine to ease removal, driver's side done in 5 mins. Had to pull the wheel and the inner fender liner on pass side to get to the pass side connector but got it changed also without any issues.

So far the improvement has been really good. Idle quality is just like new, engine has more power and after filling the tank yesterday and checking mileage, I'm back to just over 16 mpg in mostly city driving from the 13/14's.

Just thought I'd share my success since the O2's don't seem to be listed as a regular maintenance item in Ford's service protocol. I'm thinking the Techron and/or the Deep Creep wiped some coating off the O2's making them not as sensitive as before and causing the performance decline without actually setting a code.