O2 Oxygen Sensor Tips? (Mazda 3)

tunzafun001 on 14/02/2025 - 00:53
Last edited 14/02/2025 - 09:37 by 1 other user

Hi all, my partners 2007 Mazda 3 engine light came on.

Ran the diagnostics via OBD.

P0131 and P2251 - O2 sensor bank 1
Checked the wiring, fuses. Looked fine. Ordered a new 02 sensor from eBay.

She was a little stuck in the threads, so applied some CRC 66 to get it moving. Came out easy then.

Replaced it and fired the car up. A bit of smoke from sensor area (put it down to the CRC 66 burning off).

She went to the shops. Came back. Error light!

Back on the diagnostics.

P0030 and P0134 - different code, same sensor.

Did another reset. Took it for a drive. No code. Turned off, stated it back up. Engine light was instant. Codes were back.

Reset again.

Checked the sensor specs while running. Showed a flat line 1.01v (could be a limitation of my cheap obd dongle and obd fusion app). Current was 0.03mA. Blipped the throttle and it shot to 6.5mA (car spluttered, light came on).
Voltage remained dead constant at 1.01v (again, probably the dongle / app) ..maybe.

Does CRC smoke that much, or did I get a dud 02 sensor and the heating element in the new sensor went up in smoke straight away etc?

Or perhaps it is something else I have missed and not the sensor at all?

Thought it was a straight forward fix… apparently not.

• 

  AustriaBargain on 14/02/2025 - 01:27

  +5

  Your new error codes (P0030, P0134) suggest the replacement O2 sensor may have a faulty heater circuit or is completely dead. A blown fuse could also be the issue, as the O2 heater shares power with other components. Check the O2 heater fuse, wiring, and connector for damage or poor contact, especially if CRC 66 got into the plug. Also, measure voltage at the sensor plug—12V for the heater circuit (key on, engine off) and a fluctuating 0.1V–0.9V signal when running.

  If the fuse is fine and the wiring checks out, the replacement sensor is likely faulty, especially if it was a cheap eBay unit. Try swapping it for a known good brand (Denso, Bosch), clear the codes, and monitor live data with a reliable scanner. If the issue persists, the ECU or relay controlling the heater circuit may need further investigation.

  • 

    ATTS on 14/02/2025 - 10:19

    Your very knowledgeable, how do i learn more automotive stuff like you

    I know a little like

    Thermostat stuck open, hole in b pipe, little qbout alternator voltage but would like to know more. Any advice?

  • 

    tunzafun001 on 14/02/2025 - 10:51

    Yeah, will pull it in a minute.

    It's a 5 wire sensor. So will have a play with a multimeter to see what is what.

    Will leave it plugged in after removing and monitor it with a heatgun with ignition on to see if it gets hot.

    Hopefully can find the signal voltage, then cover it with a fuel rag and see some change.

    Any ideas on why the OBD has a mV reading? Service manual also says to check the sensor goes above 0.25mA under throttle (doesn't specify voltage)… Different. I thought the mA was the heater reading. But doesn't appear so.

  • 

    tunzafun001 on 14/02/2025 - 19:50

    Well, the code says a failed heater circuit.

    But it's 3 ohms, and I plugged it in outside the housing. It gets super hot.

    The code is wrong

  • 

    tunzafun001 on 15/02/2025 - 02:20

    @AustriaBargain

    This is a 5 wire wide band sensor. It is fixed voltage and measures varying current.

    It changes very quickly. I think it should be around 0.25mA and increase when you hit the throttle and deliver a rich mixture.

    This is the information I'm struggling to find. What resistance range should a Mazda 3 (2007) operate in?

    Also, I pulled it out and powered it. It got extremely hot. So the heater works…yet the ECU is saying it isn't (not looking good for the PCM/ ECU).

• 

  onetwothreefour on 14/02/2025 - 11:11

  I changed mine recently on my car with some help from Ozbargain https://www.ozbargain.com.au/node/889520, but I am by no means an expert in the area.

  In my research I found another forum recommending only replacing the sensor with a genuine factory sensor as people have had issues with using cheaper sensors. Not sure what type of sensor you used but that might be an issue?

• 

  JIMB0 on 14/02/2025 - 11:18

  Ordered a new 02 sensor from eBay.

  Either get one from a reputable source or just put a sticker over the light of it's otherwise running alright.

• 

  nizzkid on 14/02/2025 - 12:25

  +1

  Yeah, you need a OEM sensor.
  A cheap Chinese one is almost guaranteed to fail or not work correctly.

  • 

    tunzafun001 on 14/02/2025 - 12:28

    Hmm.. it's labelled for a Mazda. Has the OEM Mazda plug.

    Just tested the heater circuit on it..all good.

    But it's a wide band sensor (5 wire).

    Need to put the old one in (heater element on that is gone - so that was the problem). But the resistance readings might be ok when at temp.

    Will compare…. Fair chance..it's a rubbish sensor.

    Love working on exhaust bits…not. can't wait.

  • 

    tunzafun001 on 14/02/2025 - 15:17

    $1250 for an 02 sensor from Mazda….lol!

    • 

      JIMB0 on 14/02/2025 - 19:03

      Try a second hand one from the wreckers.

      • 

        tunzafun001 on 14/02/2025 - 21:33

        Yeah, that was my plan.

        But I now don't think it's the 02 sensor.

        Maybe the relay that drives it, or the ECU itself.

        Heaps and heaps of posts on Reddit about replacing an 02 sensor, with the same code.. but it didn't help.

        Not one of the posts has a solution though unfortunately

• 

  lgacb08 on 15/02/2025 - 08:51

  Nah, doesn't have to get it from Mazda, possibly incorrect part, for the sake of only a hundred buck just get another one. eBay sensor is fine depending from who you get it from, plenty of sellers up there selling good parts, just the matter of finding the correct one.

  • 

    tunzafun001 on 15/02/2025 - 11:33

    It's a 5 wire wide band sensor. Identical plug wiring. Can't really be a wrong part. But could be a dud.

    What I'm trying to work out is why are my OBD codes saying the heater isn't working, when I've confirmed it does?

    I put the new sensor on the loom, but old sensor in the exhaust (obviously not plugged in to anything). Started car. New sensor sitting out in the air got instantly hot.

• 

  tunzafun001 on 15/02/2025 - 13:50

  Ok, put old sensor back in. Got my partner to drive while I logged data (O2 sensor bank 1 current (only option - can't log set voltage), 02 bank 1 temp, bank 2 voltage, AFR,

  Everything is in perfect sync!

  Bank 1 leans/ richens in perfect synchronisation with post cat sensor and AFRs match. AFR perfect stoich.

  O2 temp perfect flat line (going slightly above under hard acceleration). PWM of heater clearly working.

  Basically all looks right to me.

  BUT… It throws those same codes?? One of which is low voltage bank 1 (but I can't view that). However, the current does what it should..ie. negative under acceleration, 0 amps coasting, +0.25 mA when lifting off throttle. Voltage must be right..I think?

  Strange thing was, although OBD Fusion picked up the codes (same as before) ..it didn't trigger an engine light.

  Got home. ..Codes went away without me doing any reset.

  Head is officially done in.

  Any clue what could possibly be going on here?

• 

  tunzafun001 on 15/02/2025 - 14:21

  Ok.. put old sensor back in. Got my partner to drive while I logged data (using OBD Fusion on Android) - Great free app BTW.

  Logged

  • O2 sensor bank 1 current (only option is current- can't log set voltage),
    
    • 02 bank 1 temp,
    • O2 bank 2 voltage,
    • AFR,

  Everything is in perfect sync!

  Bank 1 leans/ richens in perfect synchronisation with post cat sensor (bank 2) and AFRs match. AFR perfect stoich.

  O2 temp perfect flat line (going slightly above under hard acceleration). PWM of heater clearly working.

  Basically all looks right to me.

  BUT… It throws those same codes?? One of which is low voltage bank 1 (but I can't view that). However, the current does what it should..ie. negative under acceleration, 0 amps coasting, +0.25 mA when lifting off throttle. Voltage must be right..I think?

  Strange thing was, although OBD Fusion picked up the codes (same as before) ..it didn't trigger an engine light.

  Got home. ..Codes went away without me doing any reset.

  Head is officially done in.

  Any clue what could possibly be going on here?

• 

  tunzafun001 on 15/02/2025 - 16:56

  Put new 02 sensor in.

  This time only the same P2251 code.

  Has to be an iffy wire somewhere?

  Side stat - the data from the eBay replacement sensor is nowhere near as good as the original.

  The replacement sensor data is very jagged. Responds in huge swings 6mA, vs small changes 2mA with the original.

  Both sit at 0.02mA when cruising. But under acceleration, there is a lot of difference. Still in spec, shouldn't throw a code. But the cheaper sensor doesn't appear anywhere near as responsive.