P172C ??

Posted by: MGM2005

P172C ?? - 06/02/20 05:03 PM


The CEL on my 2005 Mercury Grand Marquis (94k miles) just went on and 3 codes popped up :

- P0457
- P0457 (same code twice)
- P172C

P0457 refers to an EVAP leak, but I can't find much information about P172C. Some sites say it's a transmission code, other a solenoid... I'm really confused.

Do you guys have any idea ?
Posted by: professorP7B

Re: P172C ?? - 06/03/20 09:25 AM

The P0457 is probably a current and pending code (some scanners show the code twice for each status.)

P172C shows up as a shift-fork code that shows a lot of hits for the Powershift transmissions when thrown into Google. The traditional automatic in your GM has no such components, so not sure where to point you with that.
Posted by: 2008interceptor

Re: P172C ?? - 06/03/20 09:28 AM

Definition of check engine code P172C:
SHIFT FORK "A" / "B" DIRECTION CONTROL VALVE STUCK ON. The only other real reference I saw to a Ford was guy with a 96 f150. He said his flashed the O/D light and shifted rough between gears.

Here's for the other code.

Here are the most common problems that will throw the code, presented in order (somewhat) from most to least likely:

Evap System Hose– The hose that carries the fuel vapor from the fuel tank to the intake manifold can begin to leak. This will cause the P0457 code. Look for either a tear or hole in it. This is one of the most common issues that causes the code.
Vapor Canister Purge Valve– One of the most common causes of the P0457 code is the vapor canister purge valve. It’s really easy to replace. There are a few clips that you need to find and remove.
Fuel Tank Sending Unit– It is possible for the gasket that attaches the fuel sending unit to the gas tank to go bad. This can reduce tank pressure and throw the code.
Fuel Cap– A bad fuel cap can cause the code to be thrown for sure. Often, you’ll get a message informing you that the fuel cap is off even when it is on. If you are getting that message, plus a P0457, it may save you a ton of time and money to replace the fuel cap and see if it goes away.
Fuel Filler Neck– There is often a gasket where the fuel filler neck connects with the gas tank. This gasket can dry out, especially with vehicles that spend their time in dry climates. When it dries out, the fuel tank can’t hold pressure anymore.
Posted by: MGM2005

Re: P172C ?? - 06/03/20 11:12 AM

Thank you so much for the detailed answers.

I did fill up the car 2 days ago and did not tight up the cap as much as I usually do. Hopefully that's the problem. I tightened up the cap and cleared all the codes to see if they will come back.

The P172C code worries me the most. The transmission does feel just a tad sluggish between 3 and 4 but I think that's just the way they are supposed to act, right ? My '99 F-150 feels exactly the same. The only time I thought I had a problem with the trans when it felt like it jerked in O/D turned out to be a misfire caused by a loose spark plug, 4k miles after the plugs had been replaced...
Posted by: 2008interceptor

Re: P172C ?? - 06/03/20 12:07 PM

I usually click mine at least 5 times just to be safe. As for the transmission if I remember right from my friends 05 mgm 3-4 is a bit sluggish compared to the other shifts. As I read it the shifting would likely be off in all gears. You'd probably get the od light flashing as well. Just to rule it out have the battery tested. Low voltage and or an internal battery short can throw all sorts of erroneous codes.
Posted by: MGM2005

Re: P172C ?? - 06/15/20 02:34 PM

So I clicked the fuel cap a few times, cleared the codes and so far so good... I did not have the battery tested yet but I will keep that in mind. Thank you!
Posted by: MGM2005

Re: P172C ?? - 07/09/20 09:40 AM

The CEL came back the day after my last post with the code P0047. I found this TSB from Ford :


Turns out it was exactly that. The fuel cap strap was trapped between the fuel cap and top of the filler neck causing the CEL to come on.