Took the vehicle for service after the usual 7000km.Parts that were changed during the service are Spark plugs, engine oil type 5W-40, Oil filter, Air cabin filter and Air filter. Two days later l got check engine on further diagnosis with OBD2 l get error P0171. The Mechanic changed the spark plugs twice however the error come back 3 days later. I have checked for loose parts on MAF and PCV Valve fault however to no avail. Find below freeze frame for error P0171.

freeze engine

freeze engine 2

  • P0171 and higher LTFT suggest a vacuum leak. Do a thorough check for leakage
    – Jupiter
    Dec 7, 2021 at 20:55

1 Answer 1


As Jupiter states in the comment, vacuum leaks are the most likely cause (particularly around the PCV valve and EGR valve). Be sure to listen for a hissing sound of air throughout the engine compartment -- around the intake gaskets, brake hose to the master cylinder, small vacuum hoses to the evap system and elsewhere.

Other possibilities:

  • PCV valve. This part may be in need of replacement.

  • Spark plugs. Verify the correct part number of the spark plugs replaced by the mechanic. Make sure the spark plug gap is correct. Check that plugs haven't been cracked, cross-threaded or torqued to the wrong value during installation.

  • Spark plug wires. Were the spark plug boots replaced with the spark plugs, as many manufactures recommend? Has there been a small amount of dielectric grease applied to the boot of the boot? Make sure spark plug wires haven't been damaged or are shorting, and are pushed onto the plugs securely. Verify that all spark plugs are firing evenly with a timing light or in-line spark tester.

  • Dirty fuel injectors/intake valves/O2 sensors. Add several bottles of fuel injector cleaner to the gas tank and drive the car at maximum acceleration on the highway at least five times. If fuel trims show improvement (i.e., drop closer to 0.0%), continue cleaning until fuel tank is near empty. I have seen this sort of problem after spark replacement, caused by the sudden improvement in combustion.

  • Gas cap. I have found that a faulty gas cap can go undetected by the car's onboard diagnostics, especially after spark plug replacement or fuel system cleaning.

  • Throttle body. Check for air leaks. It may be a likely culprit if the fuel trim values are also running high on Bank 2.

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