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When does an o2 sensor fail and how often should they be changed on an expeditio

(1999 Ford Expedition)
in Lakewood, CA on February 22, 2013
1999 170,000 miles am told the problem is plugs and wires, but they were changed 30 months and 23000 miles ago
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of WeGo2U Mobile Mechanic on February 22, 2013
Best to get a second opinion,

You didn't say what kind of problem or symptoms if any you are experiencing, and whether or not the Check Engine Light is on.... was a scanner hooked to check for codes?

To answer your question, there is no set time frame for o2 sensors to last, they should be replaced when tested bad or degraded. I have replaced them on vehicles with as little as 60,000 miles and as much as 250,000 miles. They can fail a few different ways, the heater circuit can stop working, the sensor can be slow or out of range, or "dead" meaning it does not respond at all.

In most cased - but not all - if there is a problem with the 02's there should be a code related to the o2 sensor or fuel trim. There are also many other situations that can cause a drivability problem and codes, not all codes mean that the part is bad, for example some o2 codes can be caused by a cracked exhaust manifold or intake vacuum leak, and the 02 is actually good.

The wires and plugs should last much longer than 23,000 miles unless there is a problem causing the plugs to foul (won't affect the wires) - the wires should also last longer unless they are cheap, low quality wires or rubbing against something causing physical damage to the wire.

The o2's should be tested with a labscope to verify response time and range, the scanner does NOT test them but can assist the technician in identifying if they should be tested.

If you have a misfire, and you have a 5.4L V8 it does not have wires, it has COP (coil on plug) ignition system, some common problems with the Ford COP system is the coils and the plug boots that connect the coil to the spark plug, I always recommend replacing the boots along with the plugs.

Find a good mechanic that has and knows how to use a labscope to diagnose o2 sensors and ignition system.

Hope this is helpful, good luck!

Robert Grove, ASE Certified World Class Technician
Automotive Hall of Fame 1996, State of California Certified Advanced Smog and Emissions Instructor

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on February 25, 2013
Thank you for your answer. Very helpful and informative.
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