Oxygen sensors

Will a marginal oxygen sensor cause the “Check Engine” light to come on or do you have to wait till it fails to get an indication?

Yes, a marginal sensor of any type can cause the light up the check engine light. Typically the computer checks to see if the output from the sensor is within a certain range. If the sensor spends too much time yielding output outside that range, it’s enough to displease the computer.

You have to be very careful when diagnosing specific problems that result in a Check Engine light. Most oxygen sensors will last the life of the vehicle if the proper fuel is used, the car is kept in good tune, and you don’t hammer the throttle frequently causing a rich mixture on hard acceleration. My last two went 150,000 and 210,000 respectively when I traded cars.

If the code stored is an oxygen sensor code, you should look to see if some engine or fuel factor is causing a rich or lean condition that is being read by the sensor. If you just throw a sensor at the car, it may clear up for a short time, but turn the light on again. In other words, fix the cause, not the symptom. The code will tell you rich or lean. That’s where to start, not necessarily by replacing the sensor.

Yes. A marginal O2 sensor will cause the Check Engine light to on with an OBDII engine management system. Or on vehicles built from 1996 and on.

On the old OBDI engine management system, the crosscounts of the O2 sensor could lag up to 10 seconds before the Check Engine light would come on. But on the OBDII engine management system, if the crosscounts lag on the O2 sensor for more than 1/10th of a second it will cause the Check Engine light to come on.


Is there a reason you’re asking about a marginal sensor? Has the car been scanned and led to this or is it an assumption being made the O2 is questionable because the CEL is coming on?

It would help us a lot of we knew why you are asking and what make model and year (engine?) car you have.

Trying to figure out why I can’t get better gas milage even though my Chevy PU with a 4.8L engine is in tip top condition and I drive it very gently. I keep the tires inflated to 35psi and rarely exceed 55 MPH. I’m lucky if I get 16 MPG. Truck is a 2002 with 60K on it.

Well, it’s big truck with a small engine.

If you were to drive it at a constant speed, the gas mileage would probably be pretty good. But in stop and go traffic, you’re asking that small engine to get that mass to move. And that’s where fuel mileage takes a hit.


It’s actually a regular cab,short bed, with 2WD. I also don’t normally carry any extra weight in the truck except for my 60lb black lab who thinks he owns the truck but never contributes to filling the gas tank.

You want to improve your trucks fuel mileage. Try this for some ideas: scroll to the top (or, bottom) of this page. Click on “Actual Car Information”. Scroll down about 3 pages, past “Owning?”, to “Guide To Better Fuel Economy”. See if some of these ideas will help.

You’re close enough to the EPA figures that any variance could be in the foot itself.
Another possibility is the air filter if it has not been changed recently. Depending on the environemental conditions an air filter can clog up in a hurry.
Here in Oklahoma there are certain times of the year and certain areas in which one can replace an air filter and then have to turn right around and change it again 3 or 4 weeks later.

I often have to clean my home central A/C unit twice a year. Once in the spring and again about the end of June due to milkweed and chaff from farmland harvesting.