P0141 Rear Oxygen Sensor HEATER circuit. Possible Short.

[satori nakamoto]

Just want to confirm if I understand this correctly. 

Terminal 1: Heater ground switch

Terminal 2: Signal to ecm (0-.9 voltage)

Terminal 3: Ground for SIGNAL

Terminal 4: Power (12v/Batt.) for HEATER

Is my understanding correct that there should be ZERO voltage on a ground when the circuit is off?


I'm getting battery voltage on all terminals. Key ON. Engine OFF.

Tested from the harness connector (m61) in the engine bay. Disconnected from the harness connector connected to the rear o2.

Key ON. Engine OFF.
Battery Voltage :      12.65v
Terminal  (Red) 1:    12.55v (tested to battery POSITIVE)
Terminal 2 (Wht) :   11.23v (tested to battery POSITIVE)
Terminal 3 (Blk):      12.63v (tested to battery POSITIVE)
Terminal 4 (B/W):   12.53v (tested to battery negative)

Terminal 2 approx. 1.42v less than battery voltage

Key OFF
Battery Voltage :      12.86v
Terminal  (Red) 1:    12.78v (tested to battery POSITIVE)
Terminal 2 (Wht) :   11.71v (tested to battery POSITIVE)
Terminal 3 (Blk):      12.86v (tested to battery POSITIVE)
Terminal 4 (B/W):   12.86v (tested to battery POSITIVE )

Terminal  (Red) 1:   0v (tested to battery NEGATIVE)
Terminal 2 (Wht) :  0v(tested to battery NEGATIVE)
Terminal 3 (Blk):     0v(tested to battery NEGATIVE)
Terminal 4 (B/W):  0v(tested to battery NEGATIVE)

No terminals have continuity to battery POSITIVE. All OL.
Terminals 3 and 4 have continuity to battery NEGATIVE.​​​​​​​
Continuity between Terminals 3 and 4.



Removed fuse #9 (15Amp)
OL Terminals 1 and 2
Continuity exists on Terminal 3 and battery negative
No continuity beep on Terminal 4 but now have 90-140 ohms to battery negative. Stabilized to 80 ohms after 5mins. 23 ohms with fuse plugged back in.



ECU Disconnected
Terminal  (Red) 1:    .15v drops to .03v(tested to battery POSITIVE)
Terminal 2 (Wht) :   .15v drops to .03v (tested to battery POSITIVE)
Terminal 3 (Blk):      12.86v (tested to battery POSITIVE)
Terminal 4 (B/W):   12.86v (tested to battery POSITIVE )

0.0v on all terminals to battery NEGATIVE

No terminals have continuity to battery POSITIVE. All OL.
Terminals 3 and 4 have continuity to battery NEGATIVE.​​​​​​​
Continuity between Terminals 3 and 4.


I'm assuming I have a short?

[VStar650CL]

I haven't looked at your WD, but assuming your pin ID's are correct, on most Nissan products the heaters have their own fuse. Sometimes it's one fuse for all, others have a fuse per bank or a fuse each for front/rear, but there's almost always a fuse. That should feed terminal 4, usually with the key but sometimes battery. So 4 should read near battery with key on, measured to Bat-. Terminals 2~3 have nothing to do with the heater, but 3 is signal ground, it's the only pin with a "hard" ground. It should read battery when measured to Bat+ and 0V measured to Bat-. Pin 2 is sometimes "biased" inside the ECM but should generally read 0V measured to Bat-. If it's biased it may read a voltage when tested to Bat+. The heater ground on pin 1 is an "open collector" transistor inside the ECM, so with key off it may "float" at a very low level and with key on it should measure 0V as the ECM tries to activate the heater. On older stuff the ECM simply cuts out the heater at above ~3K RPM, the ECM doesn't "PWM" the heater like modern setups.

That all said, the right way to check the circuit is with pins 1 and 4 back-probed and the car running at idle. If the heater element has power and continuity then you'll read 12V from 4 to ground and 0V from 1 to ground. If 1 to ground reads battery then you have an open wire to the ECM or the ECM output is blown. Then, raising the RPM above the limit (usually ~3K), 1 to ground should go to battery when the ECM shuts off the transistor. If 1 remains at 0V then the heater element is open-circuit. You can eliminate or inculpate the heater element by simply ohming it, it shouldn't read 0 (short) or OL (open). Most of them read about 2~4 ohms on older stuff.

[satori nakamoto]

I haven't looked at your WD, but assuming your pin ID's are correct, on most Nissan products the heaters have their own fuse. Sometimes it's one fuse for all, others have a fuse per bank or a fuse each for front/rear, but there's almost always a fuse. That should feed terminal 4, usually with the key but sometimes battery. So 4 should read near battery with key on, measured to Bat-. Terminals 2~3 have nothing to do with the heater, but 3 is signal ground, it's the only pin with a "hard" ground. It should read battery when measured to Bat+ and 0V measured to Bat-. Pin 2 is sometimes "biased" inside the ECM but should generally read 0V measured to Bat-. If it's biased it may read a voltage when tested to Bat+. The heater ground on pin 1 is an "open collector" transistor inside the ECM, so with key off it may "float" at a very low level and with key on it should measure 0V as the ECM tries to activate the heater. On older stuff the ECM simply cuts out the heater at above ~3K RPM, the ECM doesn't "PWM" the heater like modern setups.

That all said, the right way to check the circuit is with pins 1 and 4 back-probed and the car running at idle. If the heater element has power and continuity then you'll read 12V from 4 to ground and 0V from 1 to ground. If 1 to ground reads battery then you have an open wire to the ECM or the ECM output is blown. Then, raising the RPM above the limit (usually ~3K), 1 to ground should go to battery when the ECM shuts off the transistor. If 1 remains at 0V then the heater element is open-circuit. You can eliminate or inculpate the heater element by simply ohming it, it shouldn't read 0 (short) or OL (open). Most of them read about 2~4 ohms on older stuff.

Forgot to add the wd. SMH. Thank you for such a detailed response!

I did the running test but forgot to write the results. I will run them again. I believe I also have resistance on the sensor, around the 2-4 ohm range, but I will also check again. If I recall correctly the whole reason I did these tests was because I found the sensor to be fine so therefore it must be the wiring.

I know for sure I have some sort of electrical issue because Random stuff happens when I unplugged and re-plug different sensors. Specifically on the shared power line that the rear o2 heater runs on. I also attached the wiring schematic. So my assumption is that if the wire integrity is "fine" and the sensor is fine then it must either be a short or an ecu issue.

My goal doing the tests instead of back probing was to find if there is a short in the wires somewhere. I had originally thought it was rear o2 issue but after studying the diagrams I discovered it sits on a shared circuit. The p0141 has always been the only code that is thrown. My guess is because the heating element has a higher/lower? Voltage threshold/requirement? than the other sensors. When I first got the truck there were intermitent issues with the swirl control valve and iac, which I recently discovered are also in the same circuit. They would also act funny whenever I un-plug/plugged them and thier issues would stop.

If I'm correct I then need to figure out if I go left or right. If it is on the shared power line, then I will have to pull the dash, but if instead it is a short to another sensor, I will have to open the engine bay harness. I will likely will need to open the engine bay harness anyway because of the aforementioned issues. However I would like to avoid having to pull the dash out on top of doing that if I can. I'd rather only have to deal with 1 nest of wires.

I also possibly have voltage drops coming from the ecu which is a whole nother issue but perhaps may be related. I'm assuming if it is not able to read/send/recirve the correct voltages it needs it may cause dtc or other issues. Such as voltage readings that are less than they should be. I.e the 1v+ less than battery voltage on the rear o2 as well as less than 5v on reference lines for sensors.

I want to eliminate an issue (short/ecu) on terminal 1 (ecu115) so I can focus on the other side of the circuit. Not familiar with open collectors or transistors however it is my understanding that power is supplied to the o2 from the fuse/battery and the ecu activates the ground side to complete the circuit and the voltage will flow through the o2 sensor heater therefore heating it up.

[VStar650CL]

I found a resistance spec on EC-147, it's 11.4 ~ 17.4 ohms at room temperature. That's higher than a lot of the old sensors, but it means that if yours is 2 ~ 4 ohms, the ECM will definitely consider it a short.

[FrenchPople]

I'm not sure how old the sensor is, but I would just replace it. Get the OEM sensor from Nissan if it is available. Aftermarket Sensors like to read incorrectly for some reason.