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
That’s the part where you insert the key to start the car. There’s a lock gadget on the front, and an electrical switch on the back. There’s no need to pierce the wire if you don’t have 12 volts on pin c with the harness disconnected from the sensor. You’ve got to solve that problem first, before worrying about trying to test the circuit under a load.
No. From the schematic iirc, pin c is directly connected to battery voltage, through a fuse and the ignition switch. The harness-side should measure close to battery voltage anytime the key is “on”, sensor connected or not, engine running or not.
Do you have the wiring diagram for you car? If so, suggest you post a copy of the o2 sensor section here for reference. If you dn’t have the wiring diagram, and can’t find a Chilton’s or Haynes repair manual that has one, your best bet is to hire a pro to figure this out. It’s akin to trying to drive in a strange city you’ve never been before, without a map. Not going to work.
Besides for monitoring the cat performance, on some cars I think the rear o2 sensor is also used in the computer’s air/fuel mixture calculations. The forward o2 sensor is used primarily for that function, but the rear can be used too in conjunction with the forward.
I mean PIN C goes directly to the ignition (where the 12V originates from) —> and there is no fuse inbetween (which I think you mentioned before?).
What I am trying to say, is where under the hood is the wires coming out of the ignition? so I can find the same colour wire going to/ coming from Pin C??
I have a manual, this stupid website wont allow me to upload pdfs, but I will upload the image tommorrow for you guys.
Would you know of any mazda experts on here who would be able to confirm that there are no adverse effects of having a non functioning 02 sensor?
I recall you telling me something to that effect.
Thanks.
the very first hit in Google: https://www.google.com/search?q=2000+mazda+protege+wiring+diagram&client=firefox-b-1-ab&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjBvajcifbaAhWim-AKHWT3A8AQsAQIKA&biw=1753&bih=999#imgrc=w7xFba7VDAklDM:
Yes, that’s the circuit I was referring to, thanks. It’s a little difficult to follow*, but it goes: Battery positive through a 100 am main fuse, then through the IG key fuse, through the ignition switch (on and start), through the 10A engine fuse, then to the top of the o2 sensors’ heating circuit. The ECM controls the flow of current through the heater by grounding pins 93 (back sensor) and 94 (front sensor) when it wants current to flow and heat the sensor(s).
Surprisingly whoever drafted this diagram used a four-point connection, which is usually not allowed in circuit schematics. Between the ignition switch and the o2 sensors. Four point connections are disallowed by engineers not b/c they don’t work, but b/c they create confusion for those trying to interpret the diagram.
The wire colors are shown in the diagram* above posted by @thegreendrag0n . Where the wires are physically located, hard to say. Varies car to car. In any event, once you find the wire you are looking for someplace visible, then just follow it to the point where you want to test it.
Have you tested the other sensor yet, to see if it has the battery voltage present on pin c?
Note*: The B/L (EM) wire color is new abbreviation to me. I’m guessing that means black with a stripe of some color, but no idea what that color is. (Apparently it is black with a blue stripe, see below). Not sure what the EM designation means either, other than it may mean it is a shielded wire.
Update: Found this info for mazda wire colors
B-Black
O-Orange
BR-Brown
P-Pink
G-Green
R-Red
GY-Gray
V-Violet
L-Blue
W-White
LB-Light Blue
Y-Yellow
Ok at this point can we confirm that is indeed not the rear 02 sensor since we are getting 14 ohms on it?
It is most likely a connection issue?
I tried to get the Front 02 sensor connector off (to test 12V), but its tight as f. any suggestions how I should get it off?
The colour of PIN C wire is yellow with blue stripe.
I unplugged the ignition fuse, with the fuse out there is no 12V coming to PIN A —> but there is 12V with fuse plugged in.
That link says “Mazda 3” I have a protege! lol
I’ve been doing some thinking:
You see how the B/L wire in the diagram is connected to PIN C, which is connected to the ignition coils, capacitor, main fuse, main relay…so obviously 12V is hitting all those places except PINC (but it is hitting PIN A 
)
sooo if there is a loose connections it has to be between ig coil and HO2 sensor?
this image came from google search for images, link is above in the thread
it is a ton of different images there, some coming from justanswer.com, where for a small fee it is possible to get to the expert with some special knowledge on particular model… worst case you do not like the answer, you do not pay
That schematic posted above closely matches the 2000 protégé DX 1.6L schematic I’m looking at, including the wire colors.
Look at the diagram posted above. The Y/L wire is for the rear sensor, but connects to the oxygen circuit, not the heater circuit. It is the G/R and B/L wires that service the heater part of the sensor. The B/L should have battery voltage on it when the key is in the on position (even if the engine isn’t running). You may be confused about which is pin C. Until you’ve tested that at both sensors, and are certain which pin is which, no sense proceeding further.
As far as removing connectors, that’s something that varies car to car. Usually it is possible to find someplace to pry on it with a implement of some kind (small screwdriver, pick, etc) to get it to budge apart. Often there’s a little plastic locking pin that has to be pressed on, or pulled on as part of getting auto electrical connectors apart. I’ve sometimes used a hair dryer to heat a connector up a little, which if you are lucky can make it free up easier then.
Confused about PIN C? Its the top right one right?
@Deltron posted a diagram of the connector above. Remember that when you look at the face of both connectors, one face has a reversed orientation from the other. Deltron’s figure says it is for the component side, which I presume is the sensor part of the connector, rather than the wiring harness part. “view from the terminal side” means it is a face view. If I’m interpreting the diagram correctly, then on the face view of the sensor’s connector, pin c is upper right, the face view of the harness connector, pin c is upper left. You have the wire colors now, right? Make sure they are consistent with the above. Let us know when you’ve tested whether pin c on both connectors (forward and rear) are powered up or not.
Ok i am getting 12V on the top left pin, harness side of the connector, what does that mean?
You’ve got battery power to that point in the circuit. Next step is to verify the same is true when the harness is plugged to the sensor.
So I have to pierce the insulation like you said? I have a hard time getting the probe to stick into the back of connector.
The Autozone site says:
"Heater Circuit
Specific to:
Mazda Protege 1999-2003
Disconnect the HO2S connector.
Measure the resistance between the HO2S terminals C and D. Standard value:
Front sensor-3.0-3.6 ohms
Rear sensor-5.0-7.0 ohms
If not as specified, replace the HO2S.
If within specifications, check connectors and wiring between the sensor and ECM/PCM."
but it doesnt specify if I am measuring the connector or the harness? which one?
Thanks,
