2002 f-150 5.4., 186000mi


I'm getting codes p0171, p0174 &p0420. The truck runs fine, I haven't noticed any problems other than the CEL. I started last winter with P0171 and P0174 only when very cold air temp. No codes until middle of summer, then started getting just the P0420 code. Now that it has been cold a few times again I'm getting all 3 codes.

For the p0171 and p0174 codes, I have cleaned the maf, changed the fuel filter, and replaced egr tube. I have checked for vacuum leaks with carb cleaner and starting fluid, but not found any. No change still get the codes when very cold. I do believe it may be a intake gasket leak that is only a problem when very cold. I don't know if the p0420 is related to the same problem or something different.

I do know what the codes mean, but not what is causing them.
pp0171& P0174- lean banks 1&2
P0420 - cat. covert. under efficiency

Anyone have ideas whats going on? I don't want to take it to a shop and have them throw parts at it and not fix the problem. I thinking I will just keep driving it until it starts running poorly.

 

That usually doesn't work, the computer is too fast and will compensate before there is any change in RPMs that you can hear. You have to hook up a scan tool (or equivalent) and monitor either the short term fuel trims or the upstream O2 sensor voltages while you spray in order to find the source of the vacuum leak.

Most likely, the P0420 code is a result of having excessive unburned fuel dumped into the converters due to the unrepaired "lean" codes and you've probably ruined the converter due to the delay and continued operation of the truck. Don't be surprised if the other converter is also damaged to some extent but just hasn't yet failed the self tests.

 

Follow the PCV valve hose all the way to it's vacuum source and look for any damage or cracks.

 

there are two small 1/4 inch ID size hoses about 8 inches long above the drivers valve cover.. on the EGR system.. My 2001 had those two hoses get hard / brittle, and crack at the ends due to heat from the EGR... I had 171- 174 codes.

 

I'd have to chalk that up to being a coincidence. There's really no way for the EGR sample hoses to cause a mixture issue unless the DGR valve is also stuck open. They're completely isolated from the intake side of the engine and are only installed on ONE of the two exhaust banks. Besides, the PCM SHOULD have posted EGR system faults if those hoses were not intact.

 

 

Only way to be more sure the Cat has not been affected is look at the rear Ox sensor for voltage output.
It may be ok for the near future but fail later on.
Use a Scanner to monitor the rear sensor voltage changes then look at the other side to compare.
These voltages need to be within short limits as an indication the cats are working up to there capacity to store Oxygen.
When the voltage swings to much following the fronts, the PCM program logics it a Cat failure for performance.
Unless you look at both sides you will not know at the present what you have.
Maybe neither side is really good, only one side is degraded etc..
I would do this if you care enough.
This is why Scanners are used to look into these things.
The total system operation is amazing engineering,.for a reason.
The rear sensors don't do any other functions..
Good luck.

 

Thanks for letting us know how it turned out!

 

Converter degradation often takes a while to re-manifest itself after such an incident. Don't be surprised if, somewhere down the road, you find that its service life has been severely shortened.

Good luck.