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 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. In fact mean. These codes are based on exhaust oxygen measurements. A common mistake with lean codes is to replace the oxygen sensors. This could be a very expensive mistake that will not fix the problem. Especially if both codes are present, because the possibility 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 air intake hose from the MAF (mass air flow sensor). Listen for a hissing noise that may lead you to the source of the problem. Some technicians will use a propane bottle with a hose fitting 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 it’s harder to notice a change in idle. Oxygen sensor readings can be monitored with a scan tool while checking for leaks with propane, looking for higher readings when 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 reveal itself when the smoke escapes from the problem area.
aftermarket air filters Using oil on the element can sometimes damage the MAF. Excessive oil in the air cleaner can allow excess to contact the MAF sensor element or wire. This can throw off the reading, tricking the ECU into seeing more or less airflow and therefore changing the fuel/air mixture incorrectly. I once worked on a car that wouldn’t start that had a problem with a MAF. Looking at the wire in the MAF, there was a burnt piece of junk that went through the air filter. After cleaning the sensor the car ran perfectly. The ash that was on the MAF sensor wire was messing with the reading by making the mixture so rich that the car couldn’t run. After talking to the customer, he said that they had just changed the air filter. Obviously this was when some debris got into the air intake hose which was installed on the hot wire from the MAF.
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 a lean code.