So your car’s CEL (Check Engine Light) is on and you had the codes scanned at a local auto parts store. Your car has a poor trouble code P0171, P0174 or both stored in the computer, these codes are based on the oxygen sensor (O-2) readings. A lean 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 what the fault codes are. Really to mean. These codes are based on oxygen measurements in the exhaust. A common mistake with Lean codes is replacing oxygen sensors. This could be a very costly 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, intake gasket, or even a leak in the MAF (Mass Air Flow Sensor) air intake hose. Hear a hiss 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 is not as easy to check for vacuum leaks this way because the ECU (Electronic Control Unit) will quickly compensate for added fuel and it is more difficult to notice a change at idle. Oxygen sensor readings can be monitored with a scan tool while checking for leaks with propane, looking for increased readings by enriching the mix. Another way technicians can check for vacuum leaks is with a smoke test. By introducing smoke into a vacuum hose at the engine, the leak will reveal itself as smoke escapes from the problem area.
Aftermarket Air Filters Using oil in the element can sometimes damage the MAF. Excess oil in the air filter can allow excess oil to enter the MAF sensor wire or element. This can alter the reading, tricking the ECU into seeing more or less air flow and thus changing the air / fuel mixture incorrectly. I once worked on a car that would not start and had a problem with a MAF. When looking at the wire in the MAF, there was a burnt piece of trash that went through the air filter. After cleaning the sensor, the car worked perfectly. The ash that was on the MAF sensor wire was altering the reading by enriching the mixture so much that the car couldn’t run. After talking to the customer, he said the air filter had just been changed. Obviously this was when some debris got into the air inlet hose which settled on the hot wire of 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 volume of fuel. So this is one of the least likely causes of a Lean code.
