Car Parts: Oxygen Sensors

The guy tells me I have one near my catalytic converter, and one under the hood, and they both need replacing. What are oxygen sensors, are they expensive to replace, and can I do without them?

Your car will run without them, but poorly.

The cost varies widely. When I used to work at the parts store, I sold sensors that ranged in price from $15 to over $300. They are usually not hard to replace.

Worth considering if you really want to replace it with an original manufacturer part. When I replaced mine, Daihatsu wanted something astronomical for the part. The generic part place had a no-name brand one for about a fifth the price. I naturally bought the latter and it’s still working fine now, 4 years later.

Oxygen sensors (despite a misleading name) sense the presence of oxygen in the exhaust. On late model cars that meet OBD II requirements (On Board Diagnostics level II, about 1995ish or later cars here in the states) there are two Oxygen sensors. One is located in the front of the catalytic converter and a rear sensor at or behind the converter.
Earlier cars only have front sensors.
Front Sensor responds to the level of oxygen in the exhaust sends a signal back to the engine management computer. The control unit in turn varies the mixture to try and maintain the proper mixture.
Sensor detects rich -> control unit leans mixture -> sensor detects lean -> control unit enriches mixture [lather rinse repeat]
Rear Sensor located in or behind the catalytic converter acts as the “traffic cop” to ensure that the front sensor and the catalytic are doing what they are designed to. Under normal driving conditions the front sensor should swing from rich to lean on a regular basis. If the converter is working the way it should the rear sensor should move very slowly from rich to lean since the converter should have a fairly steady amount of oxygen in the exhaust. If on the other hand the rear sensor moves back and forth rapidly like the front sensor, the computer will interpret that as a faulty converter and trip a check engine light. The rear sensor also acts as a check on the accuracy of the front sensor.
What will happen if you don’t change a bad sensor(s)?
Well first off don’t plan on passing any emission tests. That very bright “Check Engine” light will see to that. Secondly the drivability, economy, and performance of the engine can be effected by bad Oxygen sensors. Lastly it is possible that a malfunctioning oxygen senor could cause additional damage to things like the converter.

I had an O[sub]2[/sub] sensor go out on my old 1987 300ZX. Well, the sensing part went, the sending part didn’t. After the car warmed up, it would send back data saying it detected no oxygen, and the mixture went WAY lean, causing horrible problems with the engine running worth a crap at all. Worked fine warming up, before the computer told the sensor to kick in. Limped from Norfolk, VA to Savannah, GA like that one Sunday. Short term fix for me: remove the sensor. Replaced it later, but it was cheap on that car. Easy to reach.

A few questions:

Who do you refer to as “the Guy”? Have you brought your vehicle to a service center/dealer? Is your check engine light on? Has your engine performance or gas milage decreased?

If your car is a ~'96 model or later, it most likely has OBDII (On Board Diagnostics system II), and must be “scanned”. This system replaces the much simpler system that allowed the vehicle owner to look for computer errors, or “codes” him/herself.

The diagnostic “code” helps the mechanic determine the cause of the problem detected by the engine management computer.

If the car has an engine light on and has already returned an O2 sensor code, chances are you are looking at O2 sensor replacement. O2 sensors, by and large, are simple two or three wire sensors that thread directly into the exhaust pipe or exhaust manifold through a flange in the pipe. Replacement is usually simple, provided that the pipe/manifold itself is in satisfactory condition.

Princhester gives good advice, in that you should consider aftermarket replacement parts.

There are a variety of other conditions that will cause the check engine light to come on, including things as simple as a loose gas cap.

O2 sensors aid the engine management computer in adjusting the fuel-air ratio when the computer is running in “closed loop mode”. You will experience much better fuel economy with the engine management computer system working properly.

O2 sensors do slowly degrade over time without necessarily setting off the check engine light- they don’t necessarily work so poorly that they throw a code, but they are certainly not tip-top.

I recently replaced the three on my Ford Ranger with Bosch replacements- I think they were about 50 bucks each. Bosch makes the OEM versions for lots of auto manufacturers anyway- you’ll definitely save money buying them vs. going to the dealership for the same exact part. Plus, Bosch has a good reputation as an aftermarket parts company.

Beyond that, you’ll need this odd socket to take them off/put them on without damaging them. I suppose you could use a crescent wrench or an open end wrench, but the socket was really easy.

There’s nothing to it- take the old one out, screw the new one in, and plug it in. That’s it.

Good info folks, you saved me a lot of Googling. First of all, “the guy”, was the guy that changed my oil in a fast-lube place today. He told me my “check engine” light was on (aren’t they all?), and he said he could hook up the computer and tell me what was wrong. So he did, and he said it was the front and rear oxygen sensors. My car seems to run fine, hot or cold, good pickup when I need it, idles smooth, gas mileage reasonable, so I am tempted not to replace anything until I am having a problem. Maybe that’s not such a good idea though, I will think it over for a couple days and probably go to the Autozone and price some replacements.

Whoa whoa whoa!! Red flags abound.

  1. Oil change guys are generally not well qualified to evaluate electronic engine management systems.

  2. If the “check engine” light was on, you’d see it and tell him. You wouldn’t need him to tell you. (I’m not sure what the “aren’t they all?” comment means. If your CEL has been on and you’ve been ignoring it, then it does make sense to record the codes, erase the code memory, and see how long it takes for the light to come back on and what codes are set when it does. It is not normal for the CEL to be on, and it is not on in the overwhelming majority of cars.)

  3. Unless he did some specific testing of the oxygen sensors, beyond hooking up the “computer” (scan tool, actually) and reading trouble codes, the diagnosis is not complete. For example, a “running rich” code could be caused by a faulty component actually causing the engine to run rich, or by a faulty oxygen sensor incorrectly reporting the engine is running rich.

It strikes me as a bit suspect that both O2 sensors would be faulty, especially with my preconceived notion of the level of expertise normally present in quick-lube personnel. If your CEL is on, I suggest having a competent shop test and evaluate the situation.

The guy at the fast lube?!

Maybe it’s different from where you are, but i’ve tended to notice those guys were trained monkeys. Last time i had someone else change my oil they stripped my bolt and overfilled my engine.

I’m with Gary T all the way on this one.
Without further work I would not condem the O2 sensors, as just hooking up a scan tool is not a diagnosis.
Further investagation by a technician qualified to work on your make of car is in order.

“The guy” re-set my CEL, so we will see how long it takes to come back on again. This guy seemed like he knew what he was doing, it wasn’t some nose-picking kid. I’m still of the ilk “don’t fix it unless it’s broke” though. I have too many bad memories of taking my car in for “routine maintenance” and having it returned with problems it didn’t have when I took it in.

Just a maintenance hint from my repair guy, who is also someone for whom I used to babysit (so he’s not trying to rip me off, because I useta change his diapers and he does NOT want to be embarrassed in front of his mechanic buds):

If you are in high water, or drive through mud, spray down the undercarriage of the car to get the dirt off the oxygen sensors. They will malfunction quite rapidly after deep water or mud exposure.