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P0171 - I have the code, now what?

I have a 1996 Ford Contour Sport 4cyl and the check engine light came on. My repair shop guy is on vacation so I went to Schuck’s and read the code. It is P0171 - system too lean (bank 1) Schuck’s suggested a $70 02 sensor but said I couldn’t return it if the light comes back on. Any suggestions on what to do and will a novice be able to fix this with a friend’s help? This car is new to me and this hasn’t happened on other cars. Even if I can’t fix it myself, I would love this info to take to the mechanic. Thanks, Teresa

Since this is a '96, I would expect that it has about 140,000 miles on it and that these are the original oxygen sensors. I believe your car has two oxygen sensors. They are identical.

At $4.50 per gallon for gasoline, I would buy a matched pair of Denso oxygen sensors on line for $50 each and put them on myself, and consider it routine maintenance. Your mileage will likely improve. Even if the problem turns out to be something else, replacing the oxygen sensors on a car that age is not a bad idea.

Your other option is to keep resetting the Check Engine light. If the problem is an oxygen sensor, eventually it will get bad enough that the computer will correctly identify the oxygen sensor as the problem, and will tell you which one it is (the front one usually fails first).

Can you hear or feel an exhaust leak anywhere between the engine and the catalytic converter? Air can leak IN through an exhaust leak and cause faulty readings that cause the engine control system to get confused.

The oxygen sensor is a reporter. The front oxygen sensor is reporting a lean running engine, to the engine computer. If it’s reporting correctly, why would you change it? It’s just doing its job. Do you believe it, or not?

Try a cheap procedure. This is from Ford Motor Company, itself. Clean the MAF (Mass Air Flow) Sensor with MAF Cleaner (from an auto parts store). The MAF is in that black tube, just pass the air filter. This is an easy job.
While you have the air intake tube off, spray inside the throttle body, and the throttle plate, with Throttle Body cleaner. Let soak. Run the engine. Results?

All excellent suggestions. I should have also suggested looking for cracks in the tube between the the MAF and the throttle body. This car is about the right age to start expecting cracked plastic.

The reason that I jumped to oxygen sensor replacement is that the age of the car makes it a candidate. Also, my experience is that when oxygen sensors get old, the first error code us rarely for the oxygen sensor. It is more often a code for mixture or catalytic converter efficiency.

Oxygen sensors are hard to test. Contrary to what you may read, you cannot really determine the health of an oxygen sensor with a digital voltmeter. To know if it is healthy, you have to put an oscilloscope on it. Since I don’t have one of those at home, when in doubt, I chuck the sensor.

Thank you Manolito and hellokit. I will try your ideas on the weekend. This car was my father’s, original owner and has 88,000 miles on it. He always took it to the dealership so I assume all maintenance was done. I’m getting 20mpg so I would like to get better mileage; maybe change the sensors.

Changing the O2 sensors is a good idea, it gives you practice and experience in changing them but it won’t fix your problem (did I state it like Click and Clack would have?).

You have a lean condition meaning you either have a vacuum leak or the MAF sensor is dirty and clean it as previously suggested. If you car stutters at idle then it’s more likely a vacuum leak. In most cars, the MAF can be disconnected and you can clear codes and run without it and see if the P0171 comes back (it will get a P0100 because you disconnected the MAF but that’s OK, it’s expected). If it comes back then it wasn’t the MAF and you have a vacuum leak, if it doesn’t come back, it’s the MAF. Also I don’t think MAF’s are that expensive on Fords (not like the $400+ on a european car). I heard Click and Clack have a dog that can sniff out vacuum leaks, that may help.