School me on Oxygen Sensors

[UMT]

1999 Impreza Outback Sport.

 

Engine light came on about a week ago and finally got to Autozone and the put the code reader on it. 1) Rich air/fuel mixture 2) Oxygen sensors 3) plugged catalytic convertor...

 

Guy who checked it for me was a Subby owner. Said first check the air cleaner. Looked at it this morning and it is fine. He said to remove the oxygen sensors and clean them, that may clear up the problem. Another guy I talked to said that was a waste of time. Lastly, pull the catalytic convertor and see if it's plugged up.

 

So, tell me about oxygen sensors? Are they able to be cleaned? If I have to replace, are the aftermarket parts any good? Do I need a special tool to pull them?

 

Checking gas mileage and thinking it may have gone down some.

 

Any and all advice appreciated.

 

Thanks,

UMT

[idosubaru]

Tell us the actual code number P0XYZ ..... the "interpretations" and suggestions by the auto parts or random places are often not that helpful...sometimes they are, but uninformed advice is easy to come by too.

 

Without knowing, my guess is you simply need a new front Oxygen sensor. Get a front Bosch O2 sensor - aftermarket is fine, some folks prefer to get them from Subaru.

 

No, you do not clean oxygen sensors, that's hysterical.

You do not need a catalytic converter, it is not clogged and a typical "guess" from someone that doesn't know what they're talking about.

[Mike104]

Tell us the actual code number P0XYZ ..... the "interpretations" and suggestions by the auto parts or random places are often not that helpful...sometimes they are, but uninformed advice is easy to come by too.

 

Without knowing, my guess is you simply need a new front Oxygen sensor. Get a front Bosch O2 sensor - aftermarket is fine, some folks prefer to get them from Subaru.

 

No, you do not clean oxygen sensors, that's hysterical.

You do not need a catalytic converter, it is not clogged and a typical "guess" from someone that doesn't know what they're talking about.

 

Good advice. Typical code on Subaru's is P0420 that can be caused by a number of things, but changing front O2 Sensor is a good start. There was on comment by someone who changed their PCV valve and got rid of the P0420 code, but I'm not sure how common that is. Either way its a cheap and much easier thing to do before changing the O2 sensor.

[ocei77]

A google will turn up a lot, but:

O2 sensors are an emissions item. They check for how much unburned fuel is in the stream by relating it to how much free O2 there is. Some Subys use two or more to monitor this. Your IOB only has 1 sensor and it reads a value which is compared with what is stord in the ECU. If it goes above this set value, it adjusts timing, usually leaning the mixture, to get the result back within limits.

You can "fool" the sensor by using a spark plug non fouler. It's basically just an extension which moves the sensor further out of the stream, reducing what it reads, thereby bringing it within paramaters(hopefully).

A search for non fouler will explain what has to be done to make the unit usable for this deed.

There is nothing to clean in a sensor, AFAIK. It reads hot exhaust gases, tho it may be possible to get coated.

A clogged cat would surely evidence itself in loss of power and at the extreme, be cherry red after running. You can unhook the exhaust manifold and other than the noise tell if there's a major difference in throttle output.

 

The sensor can be removed easily, with just an open end to hold the O2 bung on the cat (in case it has been weakened by rust) and either a socket wrench or a specific O2 socket. Idle the car for a few minutes to make it easier to remove.

Bosch(the originators of O2 sensors) is the way to go IMO. Check out oxygensensor.com for prices or even RockAuto.

 

O.

[Red92]

What is the typical life span of an O2 sensor on the Subarus?

 

My Legacy needs one too, and I was wondering if it is worth it to grab a used one (or a few) from the junkyard the next time I'm there...

[Olnick]

Bosch(the originators of O2 sensors) is the way to go IMO.

 

Check out Amazon, they have Bosch in the $40 range. Universal type--they come with a solderless connection system that worked just fine for me.

[porcupine73]

It seems to be common at the parts stores for them to give a list of parts that could be causing the problem, of course because the whole hope is that since you're there and they were nice enough to read the codes for you that you'll buy these parts.

 

It seems you have to specifically ask for the code number(s) or they don't tell you, probably since it is meaningless and confusing to most people who stop there to get it read.

[CNY_Dave]

Good advice. Typical code on Subaru's is P0420 that can be caused by a number of things, but changing front O2 Sensor is a good start. There was on comment by someone who changed their PCV valve and got rid of the P0420 code, but I'm not sure how common that is. Either way its a cheap and much easier thing to do before changing the O2 sensor.

 

If the code is P0420 there is a good bit of free stuff to try before dropping cash on a sensor.

 

P0420 is often caused by a vacuum leak (hence a bad PCV valve can trigger it) or by an exhaust leak, even an exhaust leak that is past the second 02 sensor.

 

Dave

[UMT]

Yep, the code on the slip reads: P0420

 

"Catalyst efficiency below threshhold"

 

Ok. So where do I start to try and figure out what this is?

 

Thanks,

UMT

[CNY_Dave]

Check for vacuum leaks and exhaust leaks, most P0420s are from one of those two.

 

Look at the vacuum hoses, check for cracking, hose nipples with no hose on them, hoses not going anywhere, broken plastic hose T's, listen for a whistling noise with the hood open, replace the PCV valve.

 

You can also spray a bit of carb cleaner around the intake gaskets etc to see if the idle changes.

 

Checking for exhaust leaks is easier.

 

Dave

[UMT]

Well, lo and behold, I started the car tonite and the light went out by itself. (?????? Maybe it just burnt out after being on for 2 weeks! Ha.

 

Oh, well. I'm going to check some things anyway and see if I can find anything obvious.

 

Thanks!

 

UMT

[1 Lucky Texan]

Well, lo and behold, I started the car tonite and the light went out by itself. (?????? Maybe it just burnt out after being on for 2 weeks! Ha.

 

Oh, well. I'm going to check some things anyway and see if I can find anything obvious.

 

Thanks!

 

UMT

 

It isn't unusual for O2 sensors to become 'lazy' as they get older and won't switch fast enough. Clearing a lot of codes and monitoring how quickly the come back is a good idea. I've has 'spurious' codes on my wife's Outback from a battery with a bad cell. And my youngest daughter's Impala set some weird fuel-related code. I reset the ECU and the code's been gone for months. But, just recently, she had to have a battery cable replaced for hidden coorosion - so, maybe some kinda low/marginal voltage can cause odd codes sometimes.

If your sensors are original, they are likely getting old. The codes rarely point to a part that's bad - they point to a system that is not satisfying the ECU's threshold requirements. But I bet a lot of people have dropped $800 on a converter when an $80 sensor would have been all they needed.

 

If you can also get your short/long term fuel trims read from FreezeFrame data, that 'might' show something too. But the system will try to compensate for vacuum leaks and bad/dirty MAF and other issues so, it's a great idea to start there first.

[Caboobaroo]

^^^^^^ This ^^^^^^^^

 

 

Also let it be known that of you do have a cat going bad, it will cut the lifespan of the sensors as well.

[UMT]

Well, guys!

 

The light went out by itself! For about 1 1/2 weeks. Then it came back on!

 

So, I did the Seafoam thing: 1/3 brake booster vacuum, 1/3 in the crankcase and the last third in the gas tank. (U Tube 'Seafoam' and there are several video's)

 

1 day later: light is gone. Getting MUCH better gas mileage! Car is running great with 193k on it.

 

I'm still looking at replacing the PCV valve and going to test the oxygen sensors but I'm also going to change the plugs as they are overdue.

 

Question: Alot of guys say NGK. Standard or 'Platinum?' The last set of plugs I put in were a real fancy pair by Bosch that were supposed to give me 2-3 MPG gas mileage and it never happened. Kinda soured me on paying big bucks for plugs. What makes the 'Platinum' so much better? Any experience?

 

Thanks,

UMT

[bulwnkl]

Platinum firing tips are primarily about extending service life (i.e. 50,000 mile plug change intervals rather than 15 or 30k). Strictly speaking, the much smaller firing tip of a platinum or an iridium plug reduces the amount of energy expended firing the plug, leaving more to be contained within the actual spark itself. This, theoretically, leads to fewer misfires (as caused by marginal spark for the cylinder conditions) and thus higher mpg. In reality, it is extraordinarily rare to be able to observe a mpg change in the field that is due purely to copper vs. platinum vs. iridium vs. whatever. Such a mpg increase, if observed, is almost always the result of new plugs, not plug firing tip type.