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Bad 02 sensors can caused decreased gas mileage and performance, rough idle, surging during acceleration or cruise, increased exhaust emissions, and a lean/rich running condition (which can shorten the life of the catalytic converter).

Sensors are usually diagnosed by having them hooked up to a scanner and taking note of their response times to changes in exhaust content. An old or faulty sensor will have noticeably slower switching times. With your car though (since it's ODBII), if the oxygen sensor gets bad enough the computer will complain with a CEL. Another way that might reveal secrets about your car is looking at the spark plugs. Reading them for a too lean or too rich condition can tell you if your oxygen sensors might not be up to par anymore. But don't take the results of looking at your spark plugs as solely the oxygen sensor's fault. There are other things that affect the A/F ratio.
 

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Discussion Starter #3
this is on a 92 explorer with a 4L...idles and runs bad...drivers side plugs all showing lean,passenger all normal
 

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shelbyseven said:
probably 238000
:eek: :eek: :eek:

Yes, you can safely assume that the sensor is near dead, if functional at all. OBDI systems didn't have as tight a grip on its sensors as OBDII. Heck if you unplugged the sensor you might or might not get a CEL. Anyways, I definitely recommend replacement, and do both sensors at the same time!

Now if you're having trouble deciding if it's worth the expense...Say you drive 100 miles a day round-trip of highway driving. And say you have the 21 gallon tank. Assuming that the explorer fully tuned could get 19 MPG on the highway, you'd probably only be getting 15-16 with the 02 sensors like that. So, you drive 50 miles each way, 5 days a week. That's 500 miles of driving and 315 miles to the tank. Now, going with 15 MPG and a gas price of $2.75, you spend almost $92 a week in gas. Now let's say you change those sensors and get it all tuned up. Now you're getting 19 MPG. With the same price of gas you now only spend about $72 in gas! That's $20 a week saved! So say you spend $100 total on the sensors and an hour or so of your time to replace them. In 5 weeks they'll be paid for, and you'll continue the savings long after! This is all just figurative examples but you get the point. It's probably worth it!
 

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Discussion Starter #7
what does cel stand for?...no check engine lights, friend has a unysis(sp) computer that i hooked it too. it told absolutely nothing.
 
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