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So I'm having this issue with my car and it only happened after I left it for around 2 months without driving. There is check engine code P171 saying System running too lean. I connected OBD2 reader and managed to get some test results, everything passed except this:

MID: $3c TID:$81

EVAP Monitor (0.040")

Min: 90.25Pa Max: 8,191.75Pa

Current: 49Pa

I reset the codes to see when exactly it occurs, I drove about 120 miles on it and nothing came up. All tests other than the catalyst test were complete (including evap system tests) and no code was present. Once the catalyst tests were completed, the code came back.

Could the vacuum leak be causing this code, or is it more likely to be the O2 sensor?

  • Vacuum leaks are the biggest cause of lean codes on ford products, rotted and broken vacuum lines that connect to the intake manifold. – Moab Sep 6 at 16:10
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Since you have a scan tool, check the fuel trims to see if the ECM is adding fuel to compensate for a vacuum leak. ECM can add up to 25% more fuel, which would indicate a fairly large leak in the intake. Also check the O2 sensor reading to make sure it's switching rapidly between rich and lean.

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Check the rubber intake hose for cracking. I had several tears in mine that were causing a major vacuum leak in the intake and triggering the P0171 code. The reason for the cracking was the cycles of the rubber bending back and forth during acceleration and deceleration combined with engine heat and so on. Prior to reassembly thoroughly clean the throttle body because of dirt accumulation. If your vehicle is a drive by wire system the ECM needs to be reprogrammed to recalibrate the all the associated sensors and the idle speed. The process takes about 20 minutes if you have the required tool.

One thing I have noticed about the OBD2 codes that they may not always indicate the precise issue but show you where to look and then experience comes into play.

I recommend any vehicle over 6 years old to check this as a preventative measure. In my particular case the vehicle is question was a 2012 Honda Civic.

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