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Q: I have codes P0151, P0171, and P0174. I have changed out my O2 sensor, but now I have all these codes

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First I only had codes p0151 and p0400 once I replaced those two senors after some weeks I got more codes p0151 p0174 and p0171 what could be the cause of this problem

My car has 156 miles.
My car has an automatic transmission.

A: Hey there. The codes you have downloaded si...

Hey there. The codes you have downloaded simply relate to malfunctions in the "engine management system." It appears codes were set for oxygen sensor voltage out-of-range, possible EGR (exhaust gas recirculation) problems such as a blockage, and a generic code indicating that the air/fuel ratio is too lean. Codes only give clues. Codes are never to be read conclusively as exactly which part or parts should be replaced until the fault has been inspected.

For example, with regard to the oxygen sensor "code", an exhaust leak ahead of the sensor could cause that code to be set, not to mention a faulty oxygen sensor harness connector. So you could have a perfectly good oxygen sensor reporting the "wrong" data because of a leak in the exhaust that admits unexpected additional air that the sensor "reads".

A key point is that any part referred to in a code has to be separately and carefully diagnosed to determine if the part has actually failed as opposed to a circumstance where the part (often a sensor) is just simply reporting correct data because SOMETHING ELSE has failed. This is why most expert mechanics suggest conducting Check Engine Light diagnostics on codes before moving forward with repair.

In your circumstance, the operating lean codes could possibly be caused by an air leak after the mass air flow (MAF) sensor, such as a torn intake boot, or bad intake manifold gaskets, a dirty or faulty mass air flow sensor, a continuing problem in the oxygen sensor circuit (wiring, exhaust leak), incorrect fuel pressure (or a failing fuel pump), leaking positive crankcase ventilation (PCV) valve, clogged exhaust gas recirculation (EGR) Port, and/or leaking brake booster.

Basically, all of these items, and possibly additional ones have to be checked, and each ruled in or out, in turn, until the root cause(s) are found. If you would like to have all of these codes properly diagnosed, a certified professional from YourMechanic can come to your car’s location to inspect the car.

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