Thanks for pointing that out…but we still have to confirm that it is indeed the 02 sensor
Can’t say what that 13.2 ohms means. The resistance measurement testing the sensor’s heater resistor has to be done with the connector disconnected. It proves there isn’t a dead short in the heater circuit, so better than nothing. You’ll have to figure out how to disconnect it from the wiring harness. If there is another similar connector on the car that’s more easily accessible, practice how to disconnect it on that one.
Another idea: Since you are determined not to do any repairs costing more than $100, what about the black tape method? Just put a piece of black tape over the check engine light. There’s no other symptoms than the diagnostic code, right? Neither of my vehicles even has a rear O2 sensor, and they seem to run well enough to get me where I’m going.
The connector was disconnected.
I am sure replacing the 02 sensor will be less than $100.
Thanks.
The connector was disconnected.
I am sure replacing the 02 sensor will be less than $100.
Thanks.
13.2 ohm on the sensor side with it disconnected means the heater resistor is probably ok. There’s some slight possibility I suppose it opens up and fails only when it gets hot though. That would have to be tested when it is hot, which might be difficult to do. Replacing the sensor is a bit of a gamble, but if it is only $100, might be worth a try at this point. If I were in your position however and I was willing to spend $100, I’d spend it on a pro mechanic for an hour of their time, asking them to run the appropriate diagnostics and offering your an opinion what the problem is. You can then do whatever work they suggest yourself. I’d guess that would be a $100 better spent.
It measures 13.2ohms ,. So blue L? And green red?
What diagnostic tools are required?
After skimming this thread, I don’t see anyone suggesting voltage drop tests on a loaded circuit. Your ohm checks don’t really do much to check for load related issues.
Also, you should have 12v on the heater circuit, not 5. So that’s wrong, right away. 12v feed comes directly from the ignition switch in the Run and Start position. You’ll want to see what the voltage drop from the switch to the sensor actually is.
The heater ground is PCM pulse-width modulated from pin 94; You’ll need an oscilloscope to see it. A volt meter might show 5-6v on this ground wire, because a volt meter is showing average of a small sample. The heater power comes in on pin C and the heater ground is pin D. Both heater wires should be the same color.
The o2 signal itself should be something that you can watch bouncing on a volt meter. 0.1 to 0.9 is the swing you should see.
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About all you can do as a diy’er with a volt/ohm meter is verify the +12 battery voltage is making it to the O2 sensor on the correct pin, and that the resistance of the o2 sensor heater resistor is correct. You’ve already done the latter, not sure if you’ve verified the + 12 voltage yet (on the power pin of the heater connection). That should be checked first on the harness side (with the sensor disconnected), then again with the sensor connected, by back-probing the connector. The measurements should be close to the same either way, within 0.5 volts. If that checks out too, poster @Deltron 's post above indicates you’ll probably need either an o’scope or mazda scan tool to debug this problem further. B/c the heater current apparently isn’t just on/off, but is pulsed on and off (via the ground side). I presume that’s done to get the exact temperature needed for optimum performance of the O2 sensor. I’m not sure how the computer decides when the correct temperature is reached though. Perhaps it is done by monitoring the O2 signal from the other two pins. There remains the possibility the resistor is failing when it gets hot. To test that you’d have to get the sensor up to operating temperature , then measure the resistance again. If you feel you are in over your head, the best bet probably is to either get a shop with mazda qualifications to help you out w/completing the diagnosis, or take a flyer and just try replacing the sensor.
So to test 12V ---->> I put one lead of the multimeter on C, and the other on D? the side coming from the PCM?
- I have an upstream 02 sensor that measures 16 OHMS, can I put in bullet connectors and connect it up to the existing lead on downstream 02? (and just swap in the 02 sensor?)
Thanks.
So to test 12V ---->> I put one lead of the multimeter on C, and the other on D? the side coming from the PCM?
I have an upstream 02 sensor that measures 16 OHMS, can I put in bullet connectors and connect it up to the existing lead on downstream 02? (and just swap in the 02 sensor?)
Thanks.
When the o2 sensor is being heated, 12 volts should appear between pin C and chassis ground. Pin D isn’t chassis ground. Pin D is a pulsed computer ground from the ECM according to the poster above. The voltage between pin C and pin D depends on the duty cycle of the pulse. Without an o’scope you don’t know the duty cycle, so it is pointless to measure between C and D. First test that there’s 12 volts (C to chassis ground) on the harness side of pin C (coming from the ECM). If that’s correct, connect up the sensor and do the measurement again, by back-probing the connector pin C. The chassis ground point you use remains the same. If back-probing is impossible due to the shape of the connectors, I’ve used a sharp pin to piece the insulation of the wire in cases like that myself. I dab a little rtv on the pinhole when I’m done.
I don’t know what the effect would be of swapping the sensors to which the harness is connected. Suggest not to do that unless you are certain there are no adverse effects. Whether the resistance is 16 ohm or 13 ohms shouldn’t matter. If the sensor is bad, the resistance would be an open circuit (more than 10,000 ohms) or a short circuit (less than 1 ohm).
Shoutout to Delton, hes right.
I checked point C to ground I dont get 12V.
I measured from negative of the battery to point C , with the ignition in the ON position (car not running).
So I should now trace that yellow green wire? check the fuses?
Thanks for the suggestions!
Double check that measurement w/the car running. The ECM might not power up that circuit otherwise. And you should check it both with the connector disconnected (on the harness side), and with it connected.
What is rtv? I dont think I have that?
rtv tends to be used synomously w/ silicone rubber (or caulk) although technically there may be a difference. Most auto parts stores sell a type (often branded Permatex) especially for auto repair use.
I couldn’t check the voltage with the connector connected, didn’t want to risk ruining the insulation by putting a pin in there.
But from C to ground I got 0.5V this time , with the ignition ON , and the car running.
Checked the resistance on the two black wires I get 14.4ohms
Next steps?
The resistance sounds ok. You don’t measure 12 volts from the harness side of C to the chassis ground then, with the engine running, and the connector disconnected? I think you’ve found your problem at least. hmm … from what I’m seeing here, that 12 volt to pin C comes directly from the ignition switch. I don’t see any separate fuse other than the “main” fuse. the same applies to the front sensor, so see if you got 12 volts on pin C of the front sensor. that’s what i’d check next.
My neighbor measured 12V on Pin A. ( I know not pin c) with it disconnected… So maybe it closes the circuit when the connectors are connected?
I’m trying to figure out where the ignition switch is so I can measure PinC without inserting insulation.
Also I was told that there are no adverse effects to the engine having a faulty rear 02 sensor.
Thanks
My neighbor measured 12V on Pin A. ( I know not pin c) with it disconnected… So maybe it closes the circuit when the connectors are connected?
I’m trying to figure out where the ignition switch is so I can measure PinC without inserting insulation.
Also I was told that there are no adverse effects to the engine having a faulty rear 02 sensor.
Thanks