Hey everyone, I created this account after reading through many-a-threads about P0171 codes with the c230s. I have been having this code come up numerous times in the last 2-3 years and each time I think it's fixed it comes back a few months later. I've had no symptoms almost at all with the car until an incident where the car was moving fine at higher speeds but once the rpms got low at stop lights or low speeds the car shuttered terrible and threatened to stall out. This lead to the diagnosis that the check-valve had an issue and the breather hoses were eroding. Here's a list of what's been done since the code first showed up;
(idk if it's important but the car just hit 152k miles and the issues started around 140k)

New Upstream O2 sensor
MAF replaced
Purge valve replaced
SAI hose replaced
Fuel filler neck replaced (leak found when smoked)
Replaced intake manifold gasket
Replaced Booster hose
Replaced Breather Hoses (5x)
Replaced Throttle Gasket (2x)
Replaced Fuel Injector Seals (x2)
Replaced Check valve
Replaced Supercharger gasket
New spark plugs & coils

There had been no problems since the hoses and check valve were replaced (for about 6 months) before a P0420 code was thrown last month, but I cleared it to see if it would come back on or not and it hadn't. Check light came on the other night and I just had the code pulled and again it came to P0171, so I'm at a loss. Could it be the cat going bad and causing issues? I've had a few experienced people suggest that before the P0420 code came up but I'm not entirely sure at this point because it's been to the shop twice now for the same code and it just keeps coming back intermittently. Like I said I see no changes in MPG, no stuttering/shuttering, no misfires, so I'm not sure where to look from here....

 

Is this a manual transmission or an automatic one?
Have you had the transmission fluid changed at any point in time?
That could be the culprit.

 

Have you cleaned the injectors? P0171 means the ECU tries to pour fuel into it and cannot and you already did smoke tests......

 

.

 

I haven't taken them out to clean them, but I've run seafoam through a few times, maybe I'll try that first before taking it to a shop again.

 

It's an automatic, and it has not been changed since I've gotten the car, but I'm not entirely sure what this would have to do with the fuel ratio? Could you explain?

 

In my experience, bad shifting was caused by the transmission being low on fluid. And considering your car has gone 152k without a transmission fluid change, it's a good idea to have it done.
I think I glanced over the part in your post where you talked about the P0171 code, sorry about that.
Have you tried changing the fuel filter and/or the fuel pump?

 

So the problem was someone, at some point prior to all my attempts, flooded the MAF sensor with dielectric grease and no one (including myself) ever checked that module only the filter itself.

Basically took my mechanic a can of break cleaner to get all the grease out and so far so good on the light staying off. He said all the pins were essentially connected so power and ground were touching and causing the CPU to freak out and read the fuel ratio wrong.

All in all I guess I'm glad I got everything else fixed but I do wish I checked the sensor lol

Thanks for your input everyone