My SES light came on a few weeks ago and has stayed on until the codes were cleared earlier this evening. I had trouble codes present earlier this summer, but before I could pull the codes, a new tank of fuel turned the light off. I figured it was just an emissions problem and shrugged it off.

Tonight I pulled three codes as follows:
P-0420: Catalyst below threshold
P-0171: Bank 1 lean
P-0174: Bank 2 lean

I am thinking that the oxygen sensors need to be replaced on both banks. I believe there are four on my '99 4.6 Supercab; two downstream in the cats and two upstream at the headers.

I realize that this could also be caused by a weak fuel pump, but I have not had problems starting or accelerating. I also know that leaky vacuum hoses could be the culprit. I cleaned the MAF in late May or Early June. Fuel filter was replaced in March (7,000 miles ago).

I am lead to believe the 02 sensors are the culprit because of the P-0420 code.

Thoughts?

 

Nope - O2's are 100% fine and working as they should be.

420 = exhaust/egr tube leak or bad cat.

Lean codes = Most likely your PCV tube elbow.

 

Another Vaccum leak, like what jbrew said pcv elbow

 

I'll take a look at the vacuum hoses tomorrow. Thanks for saving me $160 on all four O2 sensors.

 

No problem , this is one you really want to check out -

 

That is the one that caused my lean codes. I found one @ AutoZone for $3. It was a Dorman part # 47028. Ford wanted $38 for a hose assembly which included the part that I needed. Good luck with yours.