Three months ago the check engine light was diagnosed as EGR code. I initially thought it would be a simple replacement of the valve but the garage (A) persuaded me to also replace the EGR cooler at a total cost of $2100.
A few days ago the check engine light reappeared. I took the truck to a different, local garage (B) where they diagnosed the fault as PO1335 (I'm 99% certain that that was also the original diagnosis at the previous garage). The air filter was clogged (I cleaned it over two years ago and hadn't replaced it in over 4 years - shame on me!) so they cleaned it and cleared the code. 25 miles later there is still no check engine light.
Therefore am I correct in assuming that the clogged air filter was, or at least might have been, the actual source of the problem and therefore the EGR valve and cooler replacements may have been unnecessary?
I spoke with the manager of the original garage (A) this morning who suggested that a clogged air filter alone would be very unlikely to trigger an EGR code. Apparently they checked the air filter but didn't clean or replace it. It should also be mentioned that the truck (2006 F350 diesel dually) was - and is - running fine. He also said that it would take at least 100 miles for the check engine light to register again, whereas Garage B said 25 miles.
Obviously A will honor the warranty if the check engine light recurs but my concern is whether or not they have already proved their negligence by not taking care of the simple option first ie changing an air filter.
I'd be interested in your considered opinions, especially as I'm on the point of selling the truck.

 

A P1335 trouble code on a 6.0L diesel is a faulty position sensor contained inside the EGR valve, 100% sure on that. It wouldn't be the first bad out of box EGR valve. I've installed my share of them pretty recently. When the new valve is installed, the EGR_VP reading should be about 0.85 to 0.95 volts at key-on engine-OFF, NOT 1.XX volts. If you get any of those 1.XX volt readings after installing the new valve, it needs to be junked until you get one with the good readings.

Ford dealership technician here.

 

Thank you.
Therefore I suppose that the check engine light (P1335) will pop up again sooner or later. Would it take 25 or 100 miles or more for this to happen?
E

 

With that particular trouble code, the Check Engine light will come on sooner than later, if not immediately.

 

Yes, it did...after 40 miles or so. My hopes were raised ..but it's back to the garage tomorrow to fix it again.

 

You were spot on. The garage had to replace the EGR valve. Tx