Contour Enthusiasts Group Archives Forums General Troubleshooting Engine Code p0171?

Originally posted by Big Jim:

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This is usually not an oxygen sensor problem. This is usually a marginal or dead cat. Your emissions warranty may cover it. Your extended warranty probably does not.

Your car has four oxygen sensors. One on each bank before any cats, and one after the front cat on each bank. The oxygen sensor behind the cat is actually called a cat monitor. It tells how effeciently the cat is operating.

An oxygen sensor produces a small voltage in the presence of oxygen. 0 voltage is a lean reading. .9 volts is a rich reading. The oxygen sensor is constantly switching between 0 and .9 on a properly running engine. The cat tends to stabilize the oxygen sensor readings so the swing between rich and lean will be either much slower or nearly non existant from the cat monitor oxygen sensor if the cat is working properly. The PCM compares the front and rear oxygen sensor and screams "tilt" if the readings are switching nearly at the same speed, turns on the check engine light, and sets a code. The cat monitor system in the PCM does not run constantly. It is an intermittant test that is only done when certain conditions are met including that all other monitors have run and the the engine is well warmed up.

On California emission system cars, the cats are warranted for 7/70. On Federal emission system cars, the cats are warranted for 8/80.

With all that said, I have had that code set on my car from time to time and the cat seems to have restored itself. I have not had it on for at least a year. I have scanned the oxygen sensors and the cat monitor oxygen sensors and they are switching as expected, fast in front and very slow in rear. The cat has not been replaced. It may help that I clean the throttle body, intake manifold, combustion chambers, and injectors often and keep the ignition system and air filter is exception condition.

Let us know how things work out with this.

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