Parts Needed to make bypass:

Radio Shack Part Numbers:
# 2721434 — 1MFD Cap.
# 2711356 — PK5 1MEG 1/4 W

Introduction:
Recently, I installed a custom staight pipe for off-road use only. Since the new exhaust does not have a Catalytic Converter this generates an error code in the computer which causes the check engine light to come on. This does not really hurt anything since the code just reports that the cat is not functioning properly. However, staring at the check engine light can be a little worrisome since you do not know if anything else may be trying to trip the light! The light might also trigger an open-loop hindering performance. Not to mention, the light also shines like a sun in your face at night.

About the 02 Sensor:
Building a set of MIL Eliminators gave me a chance to learn quite a bit about the emissions system of my car. Before proceeding I would like to share that information with you so that you can become more familiar with these mysterious 02 sensors.

The 02 sensor can be considered a small batery that has an operating range of 0 to 1 volt when fully warmed up to 600F. Its voltage depends on the amount of oxygen in the exhaust stream.

All 02 sensors are vented to the atmosphere which contains Approximately 21% oxygen. The exhaust of the gasoline Powered engine typically contains up to 2% oxygen. The Sensor’s output voltage depends on the oxygen content of the exhaust stream. That is, if the exhaust has 2% oxygen, it is lean. This produces a low voltage, below .3 volts (300millivolts). If the exhaust has near 0% oxygen, it is rich. This produces a high voltage, above .6 volts (600millivolts). These voltages are sent to the computer and it reacts by adjusting the air/fuel ratio. This is commonly known as the 02 feedback system and when this system is operating it is said to be in “closed loop”. When it is not operating, meaning the computer is not reading and responding to the oxygen sensor, it is said to be in “open loop”.

Keep in mind that the computer uses all the sensors to control timing, fuel mixture and emission systems. The 02 sensor as an input is used by the computer to keep the mixture as balanced as possible. When the air/fuel ratio is “balanced” it is at 14.7 parts of air to 1 part fuel by weight. That means that for every pound of gasoline the engine burns, it will need 14.7 pounds of air. Keep in mind that oxygen occupies only 21% of the total air volume needed by the engine. The term “stoichiometric” is the term refering to the point at which the catalyctic converter can be at its maximum efficiency when converting the three major pollutants (CO, HC, NOX) into harmless emissions (CO2, H2O, N, H).

The computer can only use the sensors output information under certain conditions. First, the sensor must be hot to produce a normal signal (600F). This is why most sensors today have built-in heaters to counteract the cooling effects of prolonged idling and to achieve closed loop mode sooner during warm-up. Heating the sensor also keeps it cleaner and extends its life considerably. The heater ussually gets voltage from a constant “key-on” source like the fuel pump relay fuse. This is what the four wires on the celicas 02 sensors are. The four wires are; 02 sensor output, 02 sensor ground, 12 volt heater feed, and heater ground. On 3 wire sensors, the 02 sensor grounds through its case and doesn’t require a seperate ground wire.

Secondly, the computer is programed not to go into closed loop operation until the coolant temperature sensor tells the computer the engine is warmed up. If the system tries to go into closed loop too early in the warm up period, the leaning effect on the system would cause drivability problems and pollutants.

Thirdly, the computer is also programmed to ignore the 02 sensor at near wide open throttle conditions. Maximum power requires maximum enrichment.

Also, some manufacturers have built-in time delay. For example, on some GM models, closed loop is delayed for 1 to 2 minutes every time the car starts. This allows engine stabilization to take palce before the engine goes into closed loop.

In order to read the 02 sensor, most computers send out a certain voltage to the output terminal of the sensor. This is typically around 450 millivolts. Since we know that the sensor sends low voltage (under 300mv) when a lean condition is present and high voltage (over 600mv) when rich condition is present, the computer can count the number of times the sensor crosses the 450mv mark. Cross-counts are the number of times the 02 sensor crosses 450mv. A scanner can “look” at this for you. Even though you can’t see the numbers of cross counts without a scanner, you can use a digital voltmeter to watch the open/closed loop system operate.

After absorbing the previous information we can now see how easy it would be to create some MIL Eliminators. There are two 02 sensors on the car, one before and one after the cat. The 02 sensor after the cat is the one used to read emissions, and it is the one that we need to change to make the MIL light go off. The 02 sensor on the celica is a 4 wire type sensor. The four wires are the following:

Black Wire- 12volt heater
Black Wire- Heater Ground
Blue Wire- Output Signal (goes into ECU)
White Wire- 450 millivolts reference signal sent from the computer

We know that after the cat the mixture should be lean (because the cats function is to burn up any excess fuel that the engine did not) so all we have to do is make the Output Signal sense a low voltage (under 300mv) reading and oscillate the reading to make it look like a working sensor. This can be done via the following painfully simple circuit:

Step One:
Pull back the carpet by the fuse box on the passenger side to reveal the grey o2 sensor plug and unplug it.

Step Two:
Sever the Blue wire and bridge them back together using part (#2711356 PK5 1meg 1/4 W)

Step Three:
Splice the White wire (do not cut) and bridge it with the Blue wire using part (# 2721434 — 1MFD Cap). Make sure you bridge the blue and white wire before the Resistor coming from the ECU.

Step Four:
Reconnect 02 sensor by pluging it in, then pull the ecu 15w fuse to reset it and you now have a Catalytic Bypass.