Oxygen sensor - a hoax?

Occasionally my cars have their “check engine” lights come on, and it seems to always be the oxygen sensor. I usually just replaced it, but keep wondering how something can constantly go bad on a car so often.

I understand that oxygen sensors work before (upstream) and after (downstream) the catalytic converter to measure the oxygen levels, but do you really need them working? On the one hand, the dealers are telling me that both the upstream and downstream sensors are important and if I don’t fix them I’ll damage my fuel line, but on the other hand I have one relative saying these sensor lights are a hoax, and another telling me that the downstream one isn’t important but the upstream one is.

Long story short, my downstream oxygen sensor is not working properly according to the car’s computer, so is it something I should actually worry about, or just ignore it? If I fix it myself I can buy the part online ($50) or from the dealer ($120) or have the dealer install it for $200 labor.

The check engine light is always generated by your cars ECM (electronic control module), i.e. the computer. Plugging in a reader tells you what code caused it to come on (repair shops have these).

Most states won’t let a car pass inspection if the Check Engine light is on (and the car won’t run if you pull the bulb out of the dash). At least not if the error code is emissions.

If your car keeps killing the O[sub]2[/sub] sensor I would suspect there is a bigger problem.

The O2 sensors provide info to the ECM about how efficiently (cleanly) the engine is running. A failed sensor can stop your car from running (as happened in the UK with some contaminated fuel), make it run roughly, dump unburnt fuel out the tailpipe (which will damage the catalytic converter) and damage the fuel injection system as it tries to make adjustments based on bad data.

Follow the lights - much safer.


If you replacing them on a regular basis, you need to figure out what’s causing them to go bad. I think it’s safe to assume something in the exhaust is coming from upstream is ruining them. Well, either that or something in the wiring. (Or the dealer is screwing with you to get you to keep coming back in, like clearing the code so that it turns back on in a day or so). As for your relative, I remember when the Check Engine lights used to come on automatically, I think it was at around 20,000 miles on our Ford (pre OBD-II, I think it would have been a 92 or so). No reason, but it got you to come into the dealer.

I dunno about your specific vehicle but on my 1998 F150 when the check engine light came on and my code reader indicated it could be the oxygen sensor (actually it indicated the engine was running lean which can be due to the o2 sensor), I pulled it out and cleaned it.

Replacing the sensor is expensive - Over $100.

Cleaning it took me 30 minutes to figure out how to remove the thing, along with a $6 can of spray electrical contact cleaner (the o2 sensor has some unbelievably fragile wires on it) and a $10 set of Torx Security bits (Torx bit with a hole in the center, no idea why they didn’t use a regular Torx bit). Worked perfectly.

If it’s happening all the time and you don’t drive in a dirty environment then probably something different going on, but just a thought.

OK from the top, the malfunction indicator light (MIL) is more commonly known as a check engine light, does not light up in response to a need for routine maintenance. Despite its rather misleading name it is a malfunction indicator light.

The ECM in the car monitors a whole bunch of sensors, and their operations. Among these sensors are the oxygen sensors. Older cars had one oxygen sensor, newer cars (OBDII) have two one in front of the converter, and one behind. If you have a V6 or V8 engine you might have 1 or 2 sensors per bank, ie a total of 2 sensors on an early car, and four on an OBDII car.

So what do they do? On a proper running engine, the front oxygen sensor(s) indicates if the engine is running slightly rich or slightly lean from what has been programmed into the ECM mapping for that running condition (There is no way to hold an exact fuel mixture, so the ECM hunts just slightly rich or lean of the where it wants to be). It should work like this:
Exhaust rich -> O2 indicates rich -> ECM leans mixture -> exhaust lean -> O2 indicates lean -> ECM enriches mixture -> lather rinse repeat.
The rear sensor on an ODBII car acts as a “traffic cop” to report if the converter is actually working or not. Again in a proper working car the front sensor should be cycling rich/lean/rich/lean. However on the back side of the converter the O2 sensor should be indicating a steady (or fairly steady) signal indicating that the converter has converted the HC, CO, and NOX in the exhaust. If there is a steady signal from the rear sensor, then all is OK, if the rear sensor is cycling rich/lean/rich/lean just like the front sensor, the ECM interprets this as a bad converter. Rear sensors may also have some slight fuel trim effects. In otherwords if the front sensor is always just a little rich, it cycles OK, but its switchover point is just a little richer than it should be, the rear sensor will report this. The ECM will then adjust the fuel mixture to compensate for a front sensor that is just not quite right. I don’t think this is on all OBDII systems, but I know some systems do this.

So what happens if I say fuck it and ignore the light? Well if they do emission testing where you are, you can kiss off passing. Pretty much every emission test I have ever seen has a step where the inspector turns on the key and verifies the light comes on, then starts the engine to verify that it goes out. if that does not happen you fail.
But will I do any damage? That as they say is the 64,000 question. The answer is you could. Depending on just how and which sensor fails, you could any of the following outcomes:[ul] [li]Nothing obserable :) [/li][li]Increased fuel consumption :( [/li][li]Melt down the converter :eek: :( :eek: [/li][li]Wash down the rings from too much gas, and wear out the engine. :( :mad: :eek: :( :mad: :eek: :smack: [/li][/ul] (in the above list you may substitute signs for smiles)
Unfortunately without knowing a bunch more about the exact code being generated, and seeing the engine and the data stream out of the ECM I cannot give you an estimate of just what is wrong. If you would like to mail me your car I can fix it.*
So what causes the damn thing to go bad? Well, lots of things, and many of them are not obvious. In no particular order any of the following can either kill a sensor, or give a reading that makes the ECM think the sensor is toast[ol]
[li]Air leak in the intake[/li][li]Air leak in the exhaust (could be so small it makes no audible indication)[/li][li]bad injector(s)[/li][li]Bad Mass Airflow Sensor[/li][li]Bad ground[/li][li]Oil leak[/li][li]Silicone spray[/li][li]RTV sealants that contain silicone[/li][li]Blown head gaskets[/li][li]Oil leak[/li][li]Improper installation[/li][li]Leaded gas[/li][li]Octane boosters[/li][li]Bad thermostat[/li][li]Misfire[/li][li]Overheat[/li][li]Dielectric grease or any other type of grease on the connector to the O2 sensor[/li][li]Plus probably some other that I am not awake enough to recall[/li][/ol]
Notes on the above
!, 2. any extraneous air will cause the computer to think there is a problem. Air leaks can be either on the intake side or exhaust side of the engine. Yes an exhaust leak can draw in air and cause bad O2 sensor readings.
3. poor spray patterns, or leaky/drippy
4. If the MAF is bad but not far enough out of range to alert the ECM the failure may show up as a adaptivity code, which can lead to a O2 sensor replacement.
5. This is an electrical circuit. You have to have a good ground. Some sensors ground through the threads of the sensor, and then up to the engine block through the exhaust. If the threads got rusty (which they can) sensor readings went all to hell)
6. Sensor need a supply of outside air to reference. If oil were to leak into the reference chamber, then the sensor goes to shit. Some systems pull the reference air from inside the wires that feed the sensor. If oil gets into the harness (yes it can happen) then the sensor gets contaminated. A replacement sensor also gets contaminated.
7, 8, 9. Silicone kills oxygen sensors. Period. Silicone sprays, sealants containing silicone, and if there is any kind of head gasket leak, coolants that contain silicone will kill the sensors. Don’t use silicone spray anywhere in the engine room. If you use an RTV sealant it must say either low volatility or sensor safe of the package.
12, 13. lead also kills sensors. You probably can’t get leaded gas anymore, but some octane booster may contain lead. Lead fouls O2 sensors.
14. If the thermostat open just a bit too soon, and the engine never quite reaches operating temp, the ECM will continue to provide a little bit of enrichment. Meanwhile on the other side of the ECM the O2 is reporting a rich condition. If this goes on for too long it may be recorded as a mixture code that may interpreted as a bad O2.
15, 16. A bad misfire or overheat may damage the O2 which will then show up later as dead.
17. Can be the same as an oil leak into the harness. Contaminates the sensors from the inside.

OK, I will stop teaching now. Getting back to the OP. Damage the fuel line? Scuse me but WTF? A bad O2 reading can damage many things, but the hard plastic/steel line running from the gas tank to the engine is not one of them. :dubious:
A hoax? :rolleyes: Again WTF? Spoken like someone who technical automotive knowledge stops at “Fill er up” :rolleyes:
Front more important than the rear? Well maybe, but the systems is designed to operate with both. I cannot predict just what the limp mode is for you particular car if the rear fails.
Can you replace it yourself, is the aftermarket one as good as the factory one? Depending on the car, the aftermarket one may be fine. Some cars will not tolerate anything but the factory piece. Other don’t care. Can you do it yourself? Beats me, Can you? do you know exactly what is involved on your particular car to change the sensor? $200 is a lot of labor. It probably is not a 15 minute job. On some cars the exhaust has to be dropped to access the sensor. On one model Audi you have to remove the engine to change the O2 sensor. :eek:

*this is why I always love it in car repair threads where somebody says, just go to Auto Zone and have them read the code. The code alone without the data stream will not always give you the answer.

Exactly how often is “occasionally” on any single car? Has proper testing been done to verify that there is indeed a faulty oxygen sensor, rather than a mixture problem or electrical problem that can set an oxygen-sensor-related code?

The sensors measure post-combustion oxygen because it’s proportionate to fuel mixture (fuel to air ratio), and the sensor output tells the onboard computer whether there’s too much or too little gas going into the engine. Many times per second. The computer responds to this and other inputs by adjusting fuel injection rates, many times per second.

The upstream sensor is a critical component of this fuel management feedback. If it’s reading wrong, slugglish, or inoperative, either too much or too little gas will go into the cylinders, and fuel mileage and/or performance will suffer. Tailpipe emissions will also be negatively affected.

The downstream sensor essentially tells the computer if the catalytic converter is doing its job cleaning up the exhaust. If this sensor fails, emissions will suffer. Its effect on performance and mileage is minor compared to the upstream sensor.

None of this has anything to do with a fuel line. However, besides mileage/performance issues, there can be consequences to long-term oxygen sensor failure, up to and including serious engine damage. None of this is a hoax, and the person who thinks so is ill-informed at best. Second relative is basically right, but…

…if your state or city requires emission testing, the car will fail. And if you’re driving with the “check engine” light always on, you won’t know when something else happens that would also trigger the light - you’ve effectively rendered useless a potentially significant warning light. It’s a gamble not to fix it.

By the way, “downstream oxygen sensor is not working properly according to the car’s computer” worries me. It suggests that a code has been extracted indicating insufficient sensor response, but that testing has not been done to determine if it’s a faulty sensor or a faulty converter being reported by a good sensor. Diagnosis by parts replacement (rather than by testing) can get very expensive.

Hi Rick! Great information in your post. Thank you!

I have a question. My forward and rear O2 sensors on the right bank ar showing errors:

P200F-008 G3/4 Short (right 02 sensor, before TWC [KAT]): Short circuit in the signal line (P0131)
P200C-001 G3/4 (right 02 sensor before TWC [KAT]) Aging, correction variable exceeded: Delay time too long (P2096)

P2010-008 G3/6 (Right O2 sensor, after WC [KAT]): Short circuit in the signal line (P0137)
P200E-001 G3/6 (Right O2 sensor, after TWC [KAT]): Level is above applicable threshold. (P1999)
P200E-002 G3/6 (Right O2 sensor, after TWC [KAT]): Level is below applicable threshold. (P1999)

I replaced all four O2 sensors. I still get the errors on the right bank.

The front O2 sensor goes back an forth between 0v and 1v as it should, from my understanding. However, once the O2 sensor loop closes, the voltage on the rear drops from .45v to about .04volts in a matter of 10-15 seconds and doesn’t get much higher than .08v after that.

I’m totally stumped. I have no other errors showing up on STAR (Mercedes) diagnostics. If I had a vacuum problem, it would certainly show up.

Any thoughts on what the problem could be? A bad cat? If so, wouldn’t the rear sensor show the same voltages as the front sensor?

I checked for back pressure and there is none.

To ensure the new O2 sensors are not the problem, I swapped them out with known working O2 sensors from my other car. I still get the same errors. I checked the ground connection point where two O2 sensor ground points come together. No problems there.

thank you!

You can buy an ECB-II code reader (and re-setter) at an auto supply shop.
I’ve seen cheap units which simply display the raw code. They have a booklet with the code/meaning translations.
Get one which knows the English name of the code.

Your car may be throwing a series of codes and the mechanic is doing the addition (0105 +0237 = bad O2). The reader will cycle through each code - you can at least know the underlying reason for the “Bad O2 sensor” diagnoses.

I paid $120 at a brick and mortar Auto Zone (which used to read the code for you, until the mechanics (who were charging $80 to read it bitched)).

I am certain you can get a better price.

Based on just what you have posted I think your front sensor is deteriorated and is slow responding. (P200C)
The rear sensor is quite possibly shorted based on your voltage readings. Disconnect the rear sensor and measure the voltage on the ECM side of the connector. If the voltage is about .45V you have a shorted sensor. If however the voltage is near zero you have a harness short or a bad ECM.

Last year I bought the Autel MaxiScan MS300 on Amazon (currently price is $14.48) because my then (very old) car was throwing some codes. Worked like a charm. Don’t have that car any more, but it’s still nice to have a code reader handy.