Atredeus

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?

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paimon.soror

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

fyrstormer

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

ASH8

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.

Atredeus

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?

wazup_shorty

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

Atredeus

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

ASH8

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.

Bladecutter

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.

Atredeus

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?

Bladecutter

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.

Atredeus

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

jrx13

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.

Bladecutter

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

BC.

Gras13

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

silvsrx8

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