‘O2’ code often symptom rather than sensor fault

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This article was published 20/05/2005 (7571 days ago), so information in it may no longer be current.

Q: We have purchased a 1997 Chrysler JXI convertible with a 2.5 V6 SOHC engine and approximately 83,000 km. We had an anti-theft device and a remote starter installed on the vehicle by a very reputable dealer. We also had the radiator replaced. Since the purchase we have been continuously having a problem with the service engine light coming on always with a code indicating O2 Sensor problem. Both the upper and lower sensors have been replaced several times along with wiring changes. There has never been a problem of this nature in the past. It seems unreasonable to assume that there could be so many faulty O2 sensors installed in one car. Can you offer any suggestions?

A: A bad electrical ground between the engine and the body can cause these problems but it appears this had been looked at. The O2 sensors will set a code if the fuel mixture is too lean or too rich. I would start by looking for any air leaks in the exhaust system before the sensors. A cracked exhaust manifold or leaking joint can let air in.

Then check for a broken vacuum line on the engine that is leaning out the fuel mixture. An incorrect PCV valve can also lean out the mixture. Test the fuel pressure. Low fuel pressure may allow the car to run fair but can create the wrong air fuel ratio. A plugged, sticking or leaking fuel injector can also set an O2 code, as can a misfiring spark plug or bad plug wire. There are many things that set an O2 code. The code is often a symptom rather than an actual fault of the O2 sensor. It looks like a good place to start would be with a tune-up.

Q: My 1992 Buick Regal cruise control does not shut off when I am driving. My problem occurs when I want to slow down or stop. I can press on the brake and I can shut the cruise off at the column but the cruise continues at the speed it was set at even though I try to shut it off.

I have tried to take out the fuse that operates the cruise. I have also tried to bring the car to a complete stop by braking (the cruise control is still at full speed) I turn the engine off completely and restart it but the rpm is still way up because the cruise still does not want to shut off. This problem seems to occur more often when temperatures are cold. Can you tell me what the problem is so that it can be fixed before someone gets hurt?

A: Your Regal uses GM’s Custom Cruise III system that is a vacuum operated cruise control. This system has been used on millions of GM cars and trucks from the mid 1960s to the mid 1990s. When you activate the cruise control, an electric solenoid inside the servo unit (located under the hood) closes off a port to atmospheric air and allows engine vacuum to be trapped inside the servo. The vacuum pulls a rubber diaphragm that is connected to a throttle cable, which opens the throttle. By pulsing the solenoid on and off, the vacuum is regulated and the car is kept at the desired speed.

When you turn the system off, the solenoid is supposed to vent the system. As a backup safety feature, there is also a mechanical vacuum switch on the brake pedal that will vent the system when the brakes are applied. I have seen one or the other system fail, but never both at the same time, so I would start by looking at other problems.

The first place I would check would be the throttle cable from the servo to the throttle lever. The cable may be frayed internally or corroded. This could cause it to stick and would keep the engine running at a fast speed. Disconnect the cable at both ends and move it back and forth by hand. It should move smoothly and freely. If there is any stickiness to it, replace the cable.

If there is no problem there, check the cable that runs from the gas pedal to the throttle. It could also be causing your problem. Finally, check to see there is no vacuum in the large rubber hose connected to the servo unit when the brake pedal is pressed. I suspect that the cruise system is OK though and it is just a cable problem.