Is it an O2 sensor problem?


Asked by May 28, 2015 at 05:16 PM about the 2002 Mazda 626

Question type: Maintenance & Repair

I have a 2002 Mazda 626 with about 140k miles on it. Last winter (2014-2015), I noticed
that it was idling rough. I noticed a faint smell coming from, what seemed like my vents. It
smelled almost like a blown out candle. Eventually, when the car was idling (at stop signs,
red lights, etc.), the check engine light would flash, and as soon as I stepped on the gas
again, the check engine light would either shut off or stay steady. I was always told that if
your check engine light flashes, you better pull over or you could cause serious damage.
So, considering I was (and still am) a broke college student, I could not afford to even
have the car looked at by a mechanic, let alone to repair what was wrong (I am very naive
when it comes to car issues and, after 2 transmission rebuilds and replacements, I had
always assumed repairing a car will never cost less than $1000). Because of this "fear"
and lack of money, I stopped driving it and basically let it sit throughout the rest of the
winter, starting it up every now and then. I KNOW THIS WAS A MISTAKE!!! I know this
because about 2 weeks ago, I finally had it towed and when they started it up and towed
it, my alternator was "locked up" and busted the belt (made a god-awful sound too). This
was a $500 mistake and I have paid for it.
I gave the mechanics the codes and everything and they pretty much disregarded what I
told them because it was clear to them that the alternator and belt needed to be replaced.
Which, yes, it needed to be replaced, but they did not address my original problem of the
rough idle, etc., which is WHY I had it sitting for so long. They basically told me I was
crazy and that everything would be fine after I got the new alternator. I don't know much
about cars, but I knew they were wrong. They did not check my O2 sensor which is what I
was told could be the problem.
Before I let the car sit, I did take it to AutoZone to get codes pulled. This is what they gave
me: P0172 (air fuel ratio) and P0136 (heated oxygen sensor).
I got my car back a few days ago, and to my surprise, it ran great. Until today. I noticed it
was shifting a little harder than usual, but with the problems I've had in the past with this
vehicle, I brushed it off. Once I was almost home, the hard-shifting turned into severe
jerking and jolting and the check engine light came on. No other symptoms other than
that. I have no rough idle anymore, just rough shifting of the gears.
Basically, my question is: Do you think this is actually an O2 sensor issue? Is it possible to
have different, intermittent symptoms that stem from the same problem? Like I said, I don't
know much about cars and this whole post might not even make sense, but I'm really sick
of having my car repaired and then a month later, something else goes wrong. I have
officially put more into this car than I paid for it, but I cannot afford a new one at the time.
I'm a mess.

7 Answers Explains some. Maybe not the o2 sensor it's self, but the connection on it or circuitry to it. A faulty 02 sensor will send the wrong signal to the ECU, then one of two things happen. The ECU attempts to use that signal to set the fuel/air trim OR the ECU if it detects a fault will go to factory default air/fuel trim. The odor has nothing to do with this. But when the ECU has gone in to default, or the o2 reading id wrong, that will throw the P0172, the air/fuel trim, or ratio, same thing. Now I am going back and read all that again. I know I didn't absorb it all.

One thing at a time. true, a flashing CEL indicates a more serious problem than a steady lit CEL But that does not mean you have to - no matter where you are - shut your engine off or it will be ruined. But yours was not even flashing! It came on, and went off when you gave it throttle it went off, or stayed on. THAT is not a description of a flashing CEL. It will be a continuous on/off/on/off....not when it goes off then comes back on later . Way different scenario.


I did mention that I am very naive when it comes to car problems haha. I know I did a terrible job of explaining myself, but I am clueless and being a young woman, its hard to convince the mechanics that I dealt with that I had issues BEFORE my alternator belt busted. I was just trying to give as much detail as possible. It still can't be good that it was flashing at idle... I would assume that it should not be flashing at all. I just want to have some sort of knowledge or guidance on what route to take as far as getting it looked at again. Thank you for the advice. Again, sorry for the confusion. I am totally clueless.

1 people found this helpful.

There are rarely noticeable symptoms from a bad o2 sensor. All it does is measure the amount of oxygen in the exhaust stream before and after the catalytic convertor and plays a small role in engine settings, mostly for best emissions reductions. On many cars the o2 sensors are even ignored on cold-start. Your shifting issues now are not 02 sensor related. Would not cause that. You need to have an OBDII scan...again. If/when you do, bring the codes back here and one of us will help you

No apology necessary. But please DO get another scan.


Will do. I am taking it back to the shop that just repaired it tomorrow morning. I will keep you updated and hopefully it will help someone else. Thank you.

1 people found this helpful.

You are correct, it should not even be lit much less flashing at idle or any speed

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