What's the story with O2 (Oxygen) sensors - two mechanics give different opinions

Get the voltage specs fromt he Haynes manual for your truck and check it yourself. If it is alright and that light annoys you, pop out the bulb.

Hmmm, if the 02 light was on I'd certainly take a good look at the sensor. It could be something as simple as just resetting the computer or the sensor actually being bad.

The proper way to check an 02 sensor is to visually inspect it and then with an oscilloscope. The 02 sensor on an oscilloscope will show up as a sine wave between about .2V and .8V at somewhere in the 1 to

It is possible that the 02 sensor is bad and causing the engine to run rich and thus result in sub-optimal fuel economy.. But, I'd probably go back and ask 'em why my 02 light was still on. I get the feeling they just didn't do a thorough job.

__________________ Note: To reply, replace the word 'spam' embedded in return address with 'mail'. N38.6 W121.4

Chris, I believe (and I said "believe") if the bulb is removed, the computer will then cause the engine to run in the "Open or Closed Loop" mode--cannot remember which is which--that causes the engine to run as if it was just started, i.e. rich. Kills gas mileage. If I'm wrong, then I may learn something from you. I'm like you, tho', in understanding that such lights bother some people. A similar alternative: cover the lighted cluster area with tape. (I've even seen some 'mechanics' paint it like flat black.). If John S. is not hungry, he should have the car repaired properly. That light is there for good reason, and a good technician w/proper diag. equipment should be able to find & eliminate the cause. Just my 2 cents' worth, and often worth every dime! Have a good 'un. sdlomi

So carpros seems to be saying, test the O2 sensor and if in doubt, replace it at higher mileage.

I can't argue with that advice at 118K. You will make up for it in increased fuel mileage and drivability.

In article "sdlomi" writes: $ Chris, I believe (and I said "believe") if the bulb is removed, the $computer will then cause the engine to run in the "Open or Closed Loop" $mode--cannot remember which is which--that causes the engine to run as if it $was just started, i.e. rich. Kills gas mileage. If I'm wrong, then I may $learn something from you.

Open loop. A closed loop system is one in which something from the output is fed back to the input. In this case, the oxygen level in the exhaust is fed back to the engine computer and used to control the air/fuel ratio. An open loop system is one in which there is no feedback.

Closed loop operation allows the air/fuel mixture to be adjusted to maintain efficient operation. Open loop does not; the mixture is hard-coded. You'll burn more gas (i.e. wasting money and causing more pollution) in open loop operation.

$ If John S. is not hungry, he should have the car repaired properly. $That light is there for good reason, and a good technician w/proper diag. $equipment should be able to find & eliminate the cause. Just my 2 cents' $worth, and often worth every dime!

Ditto. If the check engine light is on because the engine is running in open loop mode due to an oxygen sensor failure, your engine is not running as efficiently as it should; you'll burn more gas. The best solution is to replace the failed oxygen sensor. The engine computer will have recorded a code to indicate what is wrong, and while engine management is extremely complex, a failed oxygen sensor should be a fairly routine diagnosis.

This should have no effect on gas mileage as the post cat O2 is only used for Catalytic converter diagnosis.

Don't you believe it.

It is only ignored at cold start, and WOT. And even that is not ALWAYS the case.

Doesn't that depend on the engine control strategy? This sounds like they are trying to use the O2 sensor only in the EPA test, and switching to open loop when they leave that regime.

Basically the O2 sensor is used to keep the mixture in line for the converter to work effectively.

Under WOT the driver is demanding maximum power and ignoring the O2 is the way to achieve this. On cold start, the O2 sensor is not yet warm enough to produce an accurate signal so it is ignored at this time also.

In article snipped-for-privacy@snet.net (ChrisCoaster) writes: $The OBDII reading was "inefficient cat - operating outside $parameters". I asked my mech what that could indicate and he $shrugged: "anything!"". He suggested a test drive and checking both $02 sensors. Turns out the post 02(after the Cat) was bad, and $replaced it. Car runs smoother now, will not know if it helped $mileage until a week from now. Does his diag. sound correct?

Sure. OBD II systems use two O2 sensors - one before the cat, and one after. The one after the cat is there to monitor how well the cat is doing its job; all of the reactions in a three-wat catalytic converter involve oxygen. howstuffworks.com has an article with more detail.

So if the upstream oxygen sensor is returning readings that seem reasonable (i.e. within the bounds of whatever the people who programmed the engine computer figure is in the normal range) but the downstream oxygen sensor is returning readings that don't seem reasonable, either on their own or in relation to what the upstream sensor says, then it makes sense to suspect either the cat or the downstream sensor.

But it shouldn't affect how the engine runs. The upstream oxygen sensor is the one that's used to monitor engine performance and adjust the mixture; the downstream one is used to monitor whether the cat is doing its job. Perhaps your mechanic did something else that helped performance? Or maybe it's something else making you perceive higher performance (different weather, different gasoline, or just the placebo effect)?

It won't ignore it under WOT until it reaches a preset RPM on every (Japanese) engine I've seen.

Well, that proves it then.