P0139 - Bad O2 sensor
 

Okay. So after getting my CEL coded at Autozone and searching around on here a bit, seems like this code could either be a bad rear O2 sensor or a bad cat. But from what I've read, there's no way to tell which one. And there's no point in dropping $100-some to get a diagnostic at the dealership. So I'm looking at buying a new O2 sensor, and if that doesn't fix it, I guess I'll get a new cat under warranty.

But my biggest question is what's the cheapest option for a reliable O2 sensor?... I saw several comments on mazmart, but looking through their site I can't find them there anymore. Anyone have other options?

Instead of purchasing anything, why don't you first just drop the cat and take a look inside ?

Grasp the catalyst firmly (when cold) and shake. If it rattles, the platinum mesh has melted.

I would have thought IF you suspect your CAT is defective then take it to a Mazda Dealer...seeing you guys have a long warranty on CATS, why not eliminate this first before the O2 Sensor.

Surely the Dealer will look at the CAT first and IF it is not CAT then you can take other action.

Either way IF you buy a new 02 Sensor and it is not the issue, but he CAT, you then have comeback to the Dealer for wrong diagnosis and I would expect them to compensate you for the 02 Sensor that you did not need to purchase.

After all you can say, "I would not have bought the 02 IF you have diagnosed the faulty CAT first."

Remember also OE CATS and Sensors are different for Series 2's.

I don't want to take it to the dealership and have them run a $105 diagnostic to tell me the cat isn't the problem. I think it's unlikely anyway, a mechanic friend assures me that the cat would throw a different code if it were actually acting up.

Either way, I'm not what you would call "mechanically inclined", so I wouldn't really know what I was looking at with the cat. If it had a huge hole in it, I'd probably catch on though.

Either way, about 3 days after the fact, the check engine light has gone off. But I do notice that I got pretty horrible gas mileage on this tank. So possibly the O2 sensor is working correctly, my car was just running rich and that's the reason for the CEL?

If you car was running rich, its just another problem to solve, have you replaced your sparkplugs? air filter?, if the code comes back again, get a replacement o2 sensor, dont be affraid of dealers (trusted) their there for a reason, they cost alot, but usualy can fix your problems without any headache

Replaced plugs/coils/wires and filter when I got the car, about 12k miles ago.

Gee your car would just be out of B2B warranty I would guess??...does not matter anyway.

I would be pretty annoyed IF the Mazda Dealer wanted to charge $105 for a diagnosis particularly IF the CAT is defective.

Personally I believe it is a matter of sweet talking to the Adviser/Writer or even Service Manager, but a lot depends on the (a) relationship you may have with said dealer....or not have.

It is worth a try, they don't charge you for asking...still free speech.:)

There is one other possible way to figure out if its the O2 sensor.
If you have another S2 owner in your area that would be willing to swap sensors with you to see if that eliminates your CEL light.

If it does, then you know what the problem is, and you can get the O2 sensor replaced under emissions warranty, IIRC.

If you lived in CO, I would swap parts with you in a minute.
If you didn't need your car as a daily driver, you could mail your part to me, and I could check it out for you.

BC.

To clarify, I bought the car with 39k miles and replaced all the tune-up parts around then. It's up to 51k now.

And yes, I assume my car is beyond any B2B warranties. I know the cat is still covered under the 8yr/100k warranty, so if I took it in and that was the root cause, it'd be a free trip for me. Problem is, that's not very likely. I'd more than likely have to pay the $88 diagnostic cost for them to tell me that it's a defective O2 sensor. Then they'd try to sell me an overpriced part and make me pay for the labor to install it (hah!).

Well a quick update - the light has come on and turned off several times in the past week. Worries me a bit, and makes me think it's more than just the O2 sensor.

So general consensus - is it worth paying the $88 for the diagnostic?... Could it be something more than the O2 sensor, like my car running rich?

Are you sure its a P0139 code?

If yes, then here's all the information about the P0139:

• The PCM monitors the rear HO2S inversion cycle period, lean-to-rich response time and rich-to-lean response time when under the open loop fuel control (fuel cut off control). If the average response time is more than the specification, the PCM determines that the rear HO2S circuit response is slow.
Diagnostic support note
• This is an intermittent monitor (HO2S).
• The MIL illuminates if the PCM detects the above malfunction condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM.
• PENDING CODE is available if the PCM detects the above malfunction condition during the first drive cycle.
• FREEZE FRAME DATA is available.
• The DTC is stored in the PCM memory.

Possible causes:
• Purge solenoid valve malfunction
• Looseness of rear HO2S
• Leakage exhaust gas
• Rear HO2S malfunction
• Fuel line pressure malfunction
• Leakage fuel
• Fuel pump unit malfunction
• AIR system malfunction
• Leakage engine coolant
• PCM malfunction

So, feel free to look over all those bits yourself, and if you can't find the cause, then you might want to inspect the rear O2 sensor, or pop off your convertor, and see if its toasted.

Or, you could pop off the sensor, and send it my way.
If it sets the same CEL in my car, then you know the answer.

I won't suggest anything, but if my car pops a CEL that is caused by the O2 sensor, something positive might work out for you. By the sounds of it, I would have to perform two drive cycles back to back to get the CEL to light up in the sensor is the cause.

Like I said before, if you have someone who lives close by you with an '09 or newer, you could swap parts with them for a couple of minutes to see if its the cause. Might cost you a drink or two, possibly lunch. If you can wait a bit, send it my way, and it will only cost you postage.

BC.

That's actually amazingly helpful. And explains why the light has been going on and off, if it's only triggering every couple ignitions.

The list of possible causes is kinda worrying, but hopefully I can short circuit a diagnosis by using one of the other S2s that are here in the Cincy area. Thanks a lot for the offer, and I might have to resort to you if the guys around here turn stubborn :P

The rear 02 sensor is just a "regular" type and not a wideband. I checked real quick online and they run about $100 or maybe cheaper if you shop around. That's for an NTK brand which is probably OEM or at least equivalent.

If your at all mechanically inclined, I would just replace that rear sensor than drive around for a few days/weeks and see if the code comes back. The sensor is almost the same price as paying for an hour of diagnostic.

Did you ever get to the bottom of your issue with the CEL?

BC.

What came out of all this was it the sensor? I'm throwing the same code.

i am also throwing the same codes..im curious to know what happened?? thread seems dead though, figured id try and revive it. cheers

Just threw a P0139 on the way back from work tonight.

61k miles, stock, oil & filter changed a few hundred miles ago, and put in a new air filter yesterday. It took a bit of wrestling with the air box to get the old one out while holding it open with one hand, so I hope I didn't mess anything up there...:uhh: Didn't hear anything crack, and on visual inspection everything seems fine, so maybe just a coincidence? Car is running as well as ever. Due for plugs and coils soon...

EDIT: Upon further inspection, found a hose-clamp loose, the one that clamps the flex-hose to the air box outlet. Tightened that up, and a day later the CEL is gone.

I'm having the same code! Guess this may be a good excuse to buy an aftermarket exhaust and just replace the whole lot of it. Anybody who changes their 02 sensor and fixes the code, let us know.

Just to throw this out there, P0139 is a common code for people who try to be smart and use a fouler on a midpipe to try to prevent a p0420. Not sure if it applies to any of you who posted though.

Hi All,

Sorry for digging up this old thread but I just had the exact same issue so I figured better to keep all the info in one spot.

I have a stock 2010 R3 that just rolled over 60K miles.

I got the P0139 code with CEL yesterday and today the CEL is gone.

When it gets a bit warmer out I'll drop my CAT and have a look.

Has anyone got to the bottom of fixing this intermittent code? Seems like quite a few people are getting them.

Thanks!

I have gotten this code intermittently and lately my gas mileage has started to suck a bit worse. I just replaced the coils, wires, and plugs for a P0300 misfire code, but I am not sure how that would mess with the rear o2 sensor beyond the exhaust being a bit rich.

I haven't noticed any coolant loss or drop in performance, so nothing seems to be wrong.

If you've recently messed with your air box, check that the accordion tube is connected properly and the hose clamps are tight. Maybe a leak around the accordion tube messes up the MAF readings enough to fail the O2 sensor rich-lean test...

I haven't done anything with the air box lately but I'll have a look. The CEL went out for a day but today it is back again. I'll poke around and check some stuff out. Seems to be a pretty common problem though with our aging S2s.

you have to replace your oxygen sensor with new one. there is no way around that.

the diagnostic will tell you the same thing as the CEL code; the cheapest option is the best one.

If replacing the sensor costs $400, and you save $10 per gas at the station, then in less than two years you'll have saved more. A year goes by quickly. Might help your engine too.