Bad O2 sensor?

[Spetz]

Hi guys,

A mechanic went over my car to check for any faults etc.

He said the O2 sensor is "dead" as it only outputs a very low voltage.
From my understanding these go from 0.1v to 1.0v?

The fuel economy is fine though and not exhibiting any of the expected symptoms.

The only things I have noted which could be from a faulty O2 sensor are:
1. The car has a slightly rough idle at times
2. Under WOT the car seems to struggle, though at less than WOT the acceleration is stronger

Could the above 2 be caused by a faulty O2 sensor?
The current one is the original one at 152,000 miles

 

[Wolf359]

You'll have to provide more details. Usually there are multiple O2 sensors. Usually on a V6, there are 4, on a 4 cylinder, maybe just 2. You have the upstream and downstream sensors. The downstream sensors don't really do much and they don't really switch much. So you have to figure out which ones he was talking about. The upstream sensors switch more often. Usually if they're bad, they would throw a code. If they get old, they can get lazy and not switch that quickly but still be within spec not to throw a code. You can change the upstream sensors and that might improve gas mileage if the old ones were slow. The downstream one isn't supposed to switch much because if the converter is doing it's job, it should be pretty steady. When they switch with the upstream ones, that means that cats are shot. Anyway, with that kind of mileage, it wouldn't hurt to change the upstream ones. O2 sensor manufacturers usually recommend between 50-80k before changing them.

 

[SHOZ]

The O2s are not used at WOT. If they are bad they set a code.

Rough idle may be dirty or in need of an adjustment plugs, or FIs and maybe a dirty throttle body or IAC if you don't have an Electronic Throttle.

 

[Linctex]

Originally Posted By: Wolf359
The downstream sensors don't really do much and they don't really switch much. So you have to figure out which ones he was talking about. The upstream sensors switch more often. Usually if they're bad, they would throw a code. If they get old, they can get lazy and not switch that quickly but still be within spec not to throw a code. You can change the upstream sensors and that might improve gas mileage if the old ones were slow. The downstream one isn't supposed to switch much because if the converter is doing it's job, it should be pretty steady. .... with that kind of mileage, it wouldn't hurt to change the upstream ones.


We must be telepathic....that's almost word-for-word what I was going to say.

 

[Linctex]

Originally Posted By: Spetz

2. Under WOT the car seems to struggle, though at less than WOT the acceleration is stronger


Look for a plugged cat converter, or plugged fuel/air filters.

.....something is choking this engine.

 

[Spetz]

Thanks guys,

The car is a 2003 Mitsubishi Diamante and only has the single pre-cat O2 sensor.

 

[ejes]

If you have no codes thrown and your just chasing down those symptoms you listed, you could run into all kinds of time and money. The easiest and cheapest thing to change out first is the PCV valve. With a car that age/mileage, it wouldn't hurt to spend the 5 bucks and change it out anyway if its never been done. I can't count how many times I've seen things like this caused by that little part after spending time/$ on ignition, etc. . (Don''t trust the rattle test)

 

[ZZman]

If the mechanic said it is bad and it has over 100,000 miles on it, change it.

 

[JC1]

Is the Check Engine light on?

If you are not experiencing any issues and you don't need to pass emissions tests, I wouldn't be changing it.

 

[Trav]

Originally Posted By: SHOZ
The O2s are not used at WOT.


It depends, wide band O2 commonly referred to as air fuel monitors (technically both narrow and wide band are) do and are used at WOT.
Most modern engines use wide bands today.

 

[Kestas]

I just looked at my friend's owners manual for his BMW, to help him with maintenance. It states to change the O2 sensor every 100K.

It might be a good idea for all cars. Not all manufacturers care about maintenance past 100K and don't bother with such recommendations.

 

[Leo99]

For $20 and an Android device you can monitor the voltages yourself and see what's up.

https://www.amazon.com/BAFX-Products-Blu...ds=obd2+scanner

 

[Trav]

Originally Posted By: Kestas
I just looked at my friend's owners manual for his BMW, to help him with maintenance. It states to change the O2 sensor every 100K.

It might be a good idea for all cars. Not all manufacturers care about maintenance past 100K and don't bother with such recommendations.


That's the way I see it but many on the board disagree. There are exceptions and IIRC the 5.4 is one of them, they can be a real bugger to even get at especially the r/side front never mind get out after almost 20 years.
I usually leave these alone unless they fail or I have to drop the pipe for a exhaust manifold change, then they are getting changed regardless of mileage.

 

[Wolf359]

Originally Posted By: Trav
Originally Posted By: Kestas
I just looked at my friend's owners manual for his BMW, to help him with maintenance. It states to change the O2 sensor every 100K.

It might be a good idea for all cars. Not all manufacturers care about maintenance past 100K and don't bother with such recommendations.


That's the way I see it but many on the board disagree. There are exceptions and IIRC the 5.4 is one of them, they can be a real bugger to even get at especially the r/side front never mind get out after almost 20 years.
I usually leave these alone unless they fail or I have to drop the pipe for a exhaust manifold change, then they are getting changed regardless of mileage.


For kicks I looked up mine and Mercedes doesn't have a recommended interval for it even though the coolant change is there at 150k. I suppose in theory if you don't any issues with fuel nor any air leaks and no emissions related failure, it might last the life of the car just like there's no change interval for catalytic converters.