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For anyone with hollowed cats or test pipes, you will, from time to time, get a check engine light (CEL, SES, MIL, etc..) since the rear O2 sensors will detect malfunctioning cats. This guide will show you how to build your own simulator to prevent the ECU from detecting the lack of catalytic converters.
Contents |
What is O2 sensor simulator?
The OBD-II cars (1996-1998) have the two O2 sensors to measure the amount of oxygen in the exhaust gas. First sensor is measuring it right after gases escape engine and this data is used to adjust fuel trim of the engine, as well as catch some faulty conditions. The second sensor is located after the catalytic converter, and is used to detect the health of catalytic converter. The ECU expects the signal from the sensor to be oscillating from below 0.4v to above 0.6v, but not above 1.2v, every few seconds when cruising.
If you install the after market downpipes with no cat (which as we all know is purely for off-road applications) the ECU will detect this and indicate the error (MIL). The ECU is quite lazy at detection, and detects this condition approximately during second long trip. You can reset the ECU to clear the error code, but it's very inconvenient, as you don't really know if the error was because of oxygen sensor or some important thing is wrong and needs to be taken care of ASAP. It's also quite annoying.
Source: http://www.mkiv.com/techarticles/oxygen_sensor_simulator
I purchased all the parts from Radio Shack and it took about 45 minutes to build. I installed it in the car, running the output to both inputs on the ECU (bank 1 and 2).
Parts List
Here are all the parts you need (minus the soldering equipment).
| Quantity | Description | Radio Shack Part Number | Individual Price |
| (2) | 5mm Red Wide-Angle LED | 276-309 | $1.49 |
| (1) | LM555 Precision Timer | 276-1723 | $1.29 |
| (1) | 8-Pin Retention Contact | 276-1995 | $0.59 |
| (1) | Wire Tap-Ins | 64-3052 | $1.99 |
| (1) | UL Recognized Hookup Wire 20AWG | 278-1225 | $4.99 |
| (1) | Project Box & PC Board Combo | 270-283 | $3.99 |
| (1) | 4.7uF 35V 20% Electrolytic Capacitor | 272-1024 | $0.99 |
| (1) | 22uF 35V 20% Electrolytic Capacitor | 272-1026 | $0.99 |
| (1) | 100K ohm 1/4W 5% Resistor pack | 271-1347 | $0.99 |
| (1) | 1M ohm 1/4W 5% Resistor pack | 271-1356 | $0.99 |
| (1) | 10K ohm 1/4W 5% Resistor pack | 271-1335 | $0.99 |
I watched the voltage through my OBD2 scanner, and it was working perfect...switching from .85v - 0v every 3 seconds. I have not had the check engine light come on yet!!!
What took a long time is making sure I had the right wires on the ECU. It turns out the Factory Service Manual has the correct Pin numbers, but not Wire colors...
Here is that information on which wires to connect to the O2 simulator output to trick the ECU. (Note these colors are wrong, but the pins are correct... not sure if my colors would match yours.. but I will get those later):
- bank 1 - pin 74 - W
- bank 2 - pin 55 - B/OR
Circuit Diagram
Components
| R1 | 100 K Ohm |
| R2 | 1 M Ohm |
| R3 | 100 K Ohm |
| R4 | 10 K Ohm |
| C1 | 4.7 uF |
| C2 | 22 uF |
| D1 | 1.7v@20mA LED |
| D2 | 1.7v@20mA LED |
Additional notes
If you use different flavors of 555 timer chip or LEDs with different parameters you will need to readjust the values of R4 and R2 to get the interval and output voltage right.
Don't attach it directly to the ECU right after assembly. Instead attach it to the battery and check the output. You should get approximately 0v/0.7v flipping about every 3.3 seconds when the car is not running, and 0v/0.9v when the car is running. The current should stay below 10mA.
One LED should be always on whenever the power is supplied. Another LED indicates when the output signal is high, so it should go on and off with the signal.
When tapping the ECU wires, triple check everything before
hooking up the oscillator. The power source should read 0v when the
key is removed, about 12.6v when they key is at ACC (accessory) and
about 14.3 when the alternator is running. The resistance between
ground wire and the body shield of the ECU should be 0 ohms. And it
would be best if you run the car and monitor the voltage of the
original oxygen sensor wire before cutting it to make sure you have
indeed got the right one. The resistance between ECU PIN
#47 and ground is about 1.3 to 1.6 M Ohm.
The original sensors should still be dangling around, or plugged
into the downpipes. The reason is that ECU also monitors the
resistance of heater circuit inside the sensor. If you want to
COMPLETELY disconnect it, you will need to measure the resistance
of the heater circuit and install the right resistor between
ECU PIN #72 and ECU PIN #31. Anyway, there is no
need to do it if you just leave O2 sensor alone and only intercept
the oxygen signal wire.
The above testing and precautions will prevent you from frying the ECU and spending major $$$$. Anyway, I assume no responsibility if you still manage to do so.
