Last Updated: 09/13/2025 @ 06:47 am

Community Member Credit: twiggy144 / khemraj1999

The rear O2 sensor (downstream of the cat) failed. I had both codes 0512 and then 0912. This lights up the SES warning lamp, but the ECM does not use any other signal from the rear O2 sensor as input to manage the fuel and ignition system in a closed-loop operation. The only purpose of the rear O2 sensor is to monitor the function of the catalyst. So I didn’t care much about having a functioning rear O2 sensor. I wanted a creative solution to turn off the SES lamp. A search on eBay resulted in an “O2 sensor simulator”.

I ordered one unit and installed it. My installation also included 2 additional 25 ohms resistors rated for 15 Watt load to simulate the O2 heater function. The ECM also monitors the heater function of the O2 sensor. So I had to take care of that error code as well to have the ECM turn off the SES lamp. I tapped in the 2 resistors in series in place of the heater wires. As an alternate to resistors, I think a simple 12V 3Watt automotive bulb ( No 194) could have simulated the heater function of the O2 sensor.

The whole installation went smooth, and the ECM switched off the SES lamp. I recommend very much the DualO2Sim. It costs much less than a genuine rear O2 sensor. Please remember this simulator works only for the rear (post catalyst) O2 sensor.

The simulator is offered at $30. A genuine O2 sensor costs at least $60. I can make a write-up. I will post it later.

Wanted to share my experience with all.

Installation of the O2 sensor Simulator

The rear O2 sensor has 4 wires (on my 1998 Maxima).

Black: low Voltage (0.1 to 0.9 V) very low amperage signal generated by the sensor itself
Grey: car chassis ground
White: the O2 sensor heater 12V positive feed ( switched on with ignition key)
White: the O2 sensor heater ground (not connected with wire 2- ground, not the same. This wire runs to fuse number 31 (1999 model year ) or 34 ( 1995-1998 model year) ).

To install the O2 sensor simulator:

Cut all 4 wires of the O2 sensor. Cut the wires close to the bad O2 sensor. That will give you some good wire length after the connector located under the driver’s seat. (On my 1998 Maxima the connector is located under the driver’s seat. This is where the O2 sensor wires emerge from under the car through a rubber grommet). Leave the bad O2 sensor in the exhaust pipe.

The sensor simulator also has 4 wires:

a – red : a 12V input to feed the simulator
b – black: ground
c – and d: 2 purple wires: O2 simulated signal ( one for each actual O2 sensor if the car is equipped with rear O2 sensors).

Use a voltmeter to identify which of the white wires above (3- or 4- ) is the +12V feed. The voltmeter will show a +12V signal with wire 3-. Connect this wire with wire a-.

Connect wire 1- with wire c- or d-. (My car only had one O2 sensor ). Leave the other purple wire unconnected if you have only one rear O2 sensor.

That will take care of ECM code 0512 Downstream oxygen sensor high voltage fault.

In my case, I also had to simulate a functioning O2 sensor heater. I used 2 x 25 ohms resistors (rated for 20 watts each) and connected in series them between wires 3 and 4. Polarity is not important for these. Under 12 V these should dissipate 1.5 Watt each, with 0.24 amps running through them. Be careful where you locate these because they get hot. Not burning hot, but hot. I tied these down under the driver’s seat. I estimated this area had enough open air to dissipate the heat. That took care of ECM code 0902 (Downstream oxygen sensor heater or circuit fault).

I have not tried but I think a simple 3Watt automotive bulb (No. 194 for example) connected between wires 3 and 4 will do the job of fooling the error code 0902.

I found the O2 sensor simulator on ebay.com Look for “O2 sensor simulator”. The supplier, Baker electronics, includes detailed electrical schematics for installation. This simulator will not detect a malfunctioning catalyst. Therefore check with your local regulations for compliance with such an installation on your car.

From khemraj1999

Thought I’d revive an old thread with some pics to help others with an o2 sim install on my 1997 Maxima. I followed twiggy144’s wire codes and o2simulator.com’s installations codes.

I used a sharp knife to slice through the plastic on the wires (DONT CUT). The o2 sensor wires are hard to work with, so I sliced the harness wire. I also left the o2 sensor in to simulate the heater signal but I cut the Sensor’s signal wire (black).

So following twiggy’s instructions:

Find the signal from the o2 sensor (my case, black)
Find ground (grey)
Find 12v. (white, tested with a test light by toggling ignition)
Connect o2 sim to them, then cut the signal from the o2 sensor.

If u need to get rid of the sim..just pull wires out and tape up the slice. (no cuts involved..except the cut from the o2 sensor signal wire..my sensor was failing so I didn’t care.