Codes P0171 and P0174 – Do Not Replace an Oxygen Sensor Before Reading This

So your car’s CEL (Check Engine Light) is on and the codes were scanned at a local parts store. Your car has a lean trouble code P0171, P0174 or both stored in the computer, these codes are based on the oxygen (O-2) sensor readings. A poor code or codes indicate that there is too much oxygen in the exhaust. Remember that parts stores have employees who mean well, but may not have the experience to interpret trouble codes. Really to mean. These codes are based on measurements of oxygen in the exhaust. A common mistake with lean codes is replacing oxygen sensors. This could be a very expensive mistake that will not fix the problem. Especially if both codes are present, because the chance of both O-2 sensors failing at the same time is highly unlikely.

Most likely the cause is a vacuum leak. A vacuum leak can be caused by a vacuum hose, an intake gasket, or even a leak in the MAF (mass air flow sensor) air intake hose. Listen for a hissing sound that can lead you to the source of the problem. Some technicians will use a propane bottle with a hose attachment to help identify vacuum leaks. With today’s computers, it’s not as easy to check for vacuum leaks this way because the ECU (Electronic Control Unit) will quickly compensate for the added fuel and a change in idle is harder to notice. Oxygen sensor readings can be monitored with a scan tool while leak checking with propane, looking for higher readings by enriching the mixture. Another way technicians can check for vacuum leaks is with a smoke test. By introducing smoke into a vacuum hose on the engine, the leak will be revealed when the smoke escapes from the problem area.

aftermarket air filters that use oil in the element can sometimes damage the MAF. Excess oil in the air filter can allow excess oil to contact the MAF sensor element or wire. This can alter the reading, tricking the ECU into seeing more or less airflow and therefore changing the air/fuel mixture incorrectly. I once worked on a car that wouldn’t start and had a problem with a MAF. Looking at the wire on the MAF, there was a burnt piece of junk that went through the air cleaner. After cleaning the sensor, the car worked perfectly. The ash that was on the MAF sensor wire was messing with the reading by enriching the mixture so much that the car couldn’t run. After speaking with the customer, he said that the air filter had just been changed. Obviously this was when some debris got into the air intake hose that was installed on the MAF hot wire.

fuel pressureit could also cause a lean condition. If the fuel filter is clogged or the fuel pump pressure is low, there could also be a higher level of oxygen in the exhaust. However, most of the time, the ECU will compensate for the reduced fuel volume. So this is one of the least likely causes of lean code.

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