BenzWorld Senior Member
Date registered: Oct 2005
Vehicle: W163 ML320, W220 S500, R230 SL600TT, R170 SLK230, W140 S500, More
Location: America the Beautiful
Mentioned: 3 Post(s)
Quoted: 48 Post(s)
The P0171 code means that, on the first bank of the engine, the fuel system is running weak or a vacuum leak exists near this side of the engine. A lean condition occurs when the engine either receives too little fuel or too much air. Could be a mass airflow sensor giving wrong readings, Common causes include:
- Intake or vacuum leaks
- Clogged or faulty Mass Air Flow sensor (MAF)
- Faulty PCV valve or other parts of PCV system
- Stuck open purge valve/solenoid
- Faulty oxygen sensor
- Low fuel pressure
- Clogged-up catalytic converter
With a good scan tool that shows the live data diagnosing the code P0171 is fairly easy. Without a scan tool, the only way to find the problem is to check the basics under the hood including: is the air filter box properly closed? Does the intake boot have any cracks? Are any of the vacuum lines or hoses disconnected?
If you have a scan tool, check the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) readings at idle and at higher rpms (Read more about fuel trims below). Normally, both the LTFT and STFT should be close to zero. If the STFT rises higher than plus 15-20% at idle, but drops at higher rpms, it's most likely you have a vacuum leak. If the STFT is close to zero at idle, but elevates to 15-20% at higher rpms, the problem could be caused a faulty mass air flow sensor, clogged catalytic converter or low fuel pressure. I would bet on the mass airflow sensor, as they get dirty after a bunch of miles and it is usually unlikely that you'll have a vacuum leak. But we are just guessing. Mine did the same thing after I cleaned the MAF yesterday, hiccuped at a red light today. BUT, I only had a little cleaner left in my can from the last three cleanings on other cars; however I did think I cleaned it pretty well. I may just spring the $80 for a new Bosch MAF on Amazon as it is a fairly good bet, and it's "only $80." If it still acts up I might get new Oxygen sensors. Not necessarily the best way to fix a problem but those 2 parts are the most likely culprits, the other one not mentioned is the fuel filter which I have hears id atop the gas tank and accessed under either the 2nd or 3rd row seats, and I'm not sure but I think that part costs about $250-300. I will research that shortly just to learn! I hope it is not an issue of clogged catalytic converters, However my ML320 I owned for several years quit running and the PO did not know what to do, sold it cheap and I found out those had melted and the molten metal clogged the catalytic converters and plugged up the exhaust, and that vehicle had 130K miles on it, so it is Possible that my cats may begin to try to clog! Time will tell...Maybe I'll call the exhaust shop that worked on my ML320 and ask them if they can measure the exhaust pressure if I bring it over to them. But, it runs so well that I have doubts that it's the cats going bad... But- possible!
Last edited by tonylinc; 08-12-2019 at 05:57 PM.