Posted this issue before but haven't had anyone been able to explain why it happens. I've got a 2004 F150 XLT with the 5.4 V8 with 150,000 miles. I had a reman engine put it at 120,000. I drive about 76 miles round trip for work. I have code P0420 and want to know the best way to diagnose whether it's the cat or the 02 sensor. New coolant was used with the reman but I have to check if the radiator was flushed.

Another issue I had last summer that may be related. When the outside temperature was above 90 degrees the temp gauge in the truck would indicate over heating but when I would pull over and turn the truck off and IMMEDIATELY turn it back on the gauge was back normal. When I would tow my boat it would climb to over heating but when I would stop it would go back to normal. I haven't had this issue this year yet but it hasn't got that hot here.

This may be a couple of separate issues or one big issue but any help would be appreciated.

 

P0402 is EGR valve (or related), isn't it?

 

The code I'm getting is p0420

 

Compare HO2S upstream HO2S11 and downstream HO2S12 switch rate and amplitude. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the downstream HO2S very slow and reduces the amplitude of those switches as compared to the upstream HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines and the downstream HO2S signal begins to switch more rapidly with increase amplitude, approaching the switching rate and amplitude of the upstream HO2S. Once beyond an acceptable limit the DTC is set.

 

Possible Causes for that code:
Use of leaded fuel
•Damaged HO2S
•Malfunctioning engine coolant temperature sensor
•High Fuel Pressure
•Damaged exhaust manifold
•Damaged catalytic converter
•Oil contamination
•Cylinder misfiring
•Downstream HO2S wires improperly connected
•Damaged exhaust system pipe
•Damaged muffler/tailpipe assembly
•Retarded spark timing
•Leaking fuel injector

 

Duh! Sorry about that. Switch a couple digits and the whole world changes!

 

I would replace your engine coolant temp sensor...

 

He may have the same troubles I had trying to find such a part on a 5.4L (before I learned it doesn't have one, but instead uses a "virtual" coolant temp sensor - the CHT) It's located UNDER the Intake Manifold between Cyl 2 & 3.


I found this good article on that little bugger


http://www.searchautoparts.com/motor...ure-cht-sensor

 

Wow... I guess I would just deal with it then..PITA

 

Yeah I've thought about replacing it but after seeing what all it entails, I might just deal with it. I'm probably not going to keep this truck much longer, my wife and I have twins on the way so we need something with a little more room