Force235

Appears this may be the problem, and it is a very common problem people are running into, myself included. I'm going to change mine out and I'll let you know...

Force235

This is a common problem on 99-04 Stangs, there must be some fix it thread out there, anyone here know?

houtex

Welcome to the forums!

Summary version:
Lean conditions are about too much air to fuel. That's it. If the car's got too much air getting past the MAF, that would cause it. If the car is getting not enough fuel out of the injectors that would cause it. Chase those. These DPFE is most likely not it. Either a cracked intake or other air inletting is happening or the fuel system is not happy, bad fuel pump, blocked filter/hose/pipe, bad regulator, bad return, bad injectors (not opening/clogged.) This will require investigation and diagnostics and maybe a mechanic if you don't have the tools/knowhow/desire to do it.

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Long winded (aka my standard) version:
https://www.engine-codes.com/p1151_ford.html
The two codes are identical except one is for bank 1 (passenger side) and the other is for bank 2 (driver). That you have it on both does indicate there's a centralized cause.

Reviewing the most likely candidates:

• Faulty Upstream Oxygen Sensor Bank 2
• Fuel pressure high or low
• MAF sensor dirty/defective
• Vacuum leak on engine
• Leaking fuel injector or fuel pressure regulator
• Faulty Powertrain Control Module (PCM)

You'd have to do proper diagnostics to figure it out, or start firing parts at it which can get expensive. Besides cleaning the MAF or checking upon it, checking fuel pressure is easy to do. Those would be first. I would then do a smoke test on the intake and see if smoke gets out where it shouldn't on the intake side of things, including the gas tank as it's part of the intake in a way due to the emissions stuff. This requires a smoke machine and may require you to visit someone to either borrow, buy or hire this thing. Since it's lean, it won't necessarily be a leaking injector or regulator, but maybe... If it can't get fuel it'll toss these codes, but one would think other codes might light up.

As far as the DPFE... I'm sayin' no. It could maybe possibly be incorrectly reporting EGR stuff, true. It's a 25 buck(ish) part 'round here, so it's not without merit to parts cannon the thing into the car and see what happens, but I somehow don't think it's gonna fix the problem.

That is all being said with the understanding that I do NOT know more than you've told us, so I can only operate from the idea that you've done nothing else to try to fix this problem. If you've done everything else, throw the part in there and see, as it's about all you can do at this point beyond taking it to someone like Eric O at South Main Auto who is a dang wizard at this sort of weirdness. Or Rainman Ray maybe, he's pretty good at this stuff too. Someone like that who has the tools/knowhow and will save your hair, time and pocketbook in the long run.

Hope that helps, good luck!

Force235

@Houtex - Thanks much, good explanation and very helpful. This is what I've found since researching what these codes can be, and your answer.

The car is relatively stock aside from new BBK CAI and TB, plus Borla catback. and low miles 18k on a 2002. I decided to take the CAi off and inspect everything carefully. I found a few problems,
1. CAI pipe has hole in it for temp sensor, the temp sensor is a separate sensor on some years between 99-04 not sure which ones. The hole is supposed to be plugged, the previous owner apparently used duct tape to seal the hole, there was tape residue all around the hole but the tape was long gone. I plugged the hole properly.
2. The PCV was not installed properly, was only half turned in and oil had been leaking around the base on to the Volve cover.
3. The hoes from the PCV to the TB was incorrect size and loose at the PCV valve, leaking about a 1/32" (0.5mm) all around the diameter, small but a leak
4. The clamp used on the CAI rubber boot to the TB was also atr its smallest point, which only created a semi-snug fit boot-to-TB, I could slide if off without loosening the clamp.
5. Small red vacuum hose (plastic ) on passenger side appeared kinked due to repositioning when CAI was installed

I had previously doublechecked the O2 sensors, and they were good. I made all the necessary changes to fix the above problems, and reset the CEL. I have about 25 miles on the improved setup, and hoping no more CEL.