o2 sensors

yes i cut my catalyic converter, and the sensors in the fron and back og the pipe, which i think are the o2 sensors, are not reading back pressure, which made my check engine light come on, i red that if i put the sensors in aluminum foil that it would prevent the light coming on, does this work, if so, how would i go about taking them out, and putting them in the aluminum foil. would i cut the wire, or wouldi unbolt them, what would i put in the place of the sensor, any info on this please respoond thanx!

The O2 sensors and back pressure have absolutely nothing to do with each other :) The O2 sensor simply measure the relative O2 content in the exhaust gasses. They dont care if your cat is there or not, they will still function the same. The problem you are having is the rearward sensor is telling the ECU that the cat is not working, because in fact its not there. What people do is remove the rear O2 sensor from the pipe, but leave it plugged in. You wrap it in foil since it is heated and is a fire/burn hazard. Without going into too much detail, this tricks the ECU into thinking the cat is working. I can explain it though if anyone is curious.

Its also possible that the sensor(s) is bad. Do you know for sure what the code is for that check engine light? If it is the cat (or lack of one) the code description will be "catalyst system efficiency below threshold (bank1)" or similar.

Since I see this post here, I'd also like to ask something about O2 sensors.

What exactly is this? As I read somewhere else, it can somehow influence on gas consumption, especially if it's bad. Where is it positioned? Do you know the page in Chilton's book where I can read more?

I believe that I might check it since I do from 260-300 miles per tank (city/highway) with my A/T 1997 GS Eclipse. I think this is too much for normal driving, even with A/T transmission.

Any info would be much appreciated.

An o2 sensor just measures the partial pressure of the oxygen in the gas its exposed to, determining how much o2 is in it. The signal is then fed to the ECU. The front O2 sensor (precat) is the one the ECU uses for determining how much fuel to add. The sensor has a very narrow range (as opposed to "wideband" sensors) and is most accurate right around stoich, or 14.7 parts air to 1 part fuel (14.7:1, or roughly the point where all o2 and all fuel is consumed). The ECU doesnt shoot right for 14.7 since it doesnt have that much control, instead it cycles up and down around 14.7, or about .5 volts on the sensor (it has a 0 to 1 volt range). The average of these cycles/fluctuations will be around .5 volts, or 14.7:1 under light load.

In order to keep the O2 sensor voltage cycling around stoich, the ECU will adjust its fuel trims. This is just a parameter the ECU can use to control fuel. There is a short term fuel trim (STFT) that adjusts constantly. The running average, or decaying average, of the STFT is then fed into the long term fuel trim value (LTFT) which changes much more slowly. It wont adjust until a few conditions are met. Car has been operating at steady state for some time (idling or cruising on the highway for example), up to temperature (this is why a bad coolant temp sensor will screw up your fuel mileage, choke will always be on), you are not trying to make some power (full throttle, climbing a hill, etc), etc. You'll hear people refering to trims when talking about dialing in bigger injectors with a fuel tuning device like the SAFC, emanage, DSMlink, etc. They arent used in full throttle (WOT) tuning though.

The o2 sensor is only used in closed loop operation, and not in open loop operation. Closed loop operation is just what it sounds like. There is a closed "loop" between the sensor, the ecu, and its output to the injectors, which is then seen by the sensor, and the loop continues. Its also known as a feedback loop. Open loop is the opposite. The ECU no longer cares what the O2 sensor reports back, so there is no feed back loop. The loop is "open" (like an electrical circuit). It uses mass airflow (flow speed, baro pressure, and temp (turbo cars), or MAP (manifold absolute pressure) combined with known intake volume etc in the non-turbo cars), and a few other parameters like rpm, to pull a number off of a lookup-table and inject that predetermined amount of fuel. This is why we use an SAFC or similar to intercept and alter the airflow reading sent to the ECU, to change how much fuel delivery we actually get, thereby changing our air:fuel ratio.

The second O2 sensor, post cat, is there just to verify that the cat works. The front O2 will see that cycling we talked about above around .5 volts. If the cat is there and working, it will be almost a flat line, or constant voltage, from the second sensor. The cat smooths out those fluctuations in the O2 content of the exhaust gas mixture since it uses it to burn up any unburned hydrocarbons. If the cat is not working, or we have removed it, the rear sensor will pick up exactly the same cycling/fluctuating voltage that the front sensor reports. Then the ECU will watch it for some time, and then say ok its not working or not there, heres a "catalyst system efficiency below threshold (bank 1)" DTC, or Diagnostic Trouble Code, hich will trigger a Check Engine Light. (not all DTCs trigger CELs).

This is specific to a turbo dSM, but it should hold true in all OBD2 equiped cars, or basically any car produced for the 96 model year and later. Some earlier cars, like the 95 DSMs had switched over earlier. Note that the 1g cars do not use the rear sensor, it wasnt a requirement for OBD1 or other proprietary systems, but is for the OBD2 protocall. This is a basic overview but should be complete enough to explain how the system essentially works. :) Hope it helps.

Good info, i'll add it to my contents, and hopefully we can get some sticky action going soon. Nobody replied back to my PM's so I don't know whats up. A few questions though:

A site for my friend's WRX on his A/F gauge said to tap into the rear (on cat) o2 sensor, where it seems to me like the front o2 sensor would be one to read from... which do we use?

Is open/closed loop the same as when the car is in a choke (cold engine) situation? Which would mean that the o2 sensors are useless after warm up? If you can run a dsm without the front o2, how does the car idle in closed loop, where it can't get info for the lookup table? I've heard you can remove o2 sensors, but maybe that meant "remove wideband and put the stock back in when not tuning".

Thanks for the help :)

The second o2's CEL can be taken care of with a resistor...
Go to radioshack (or wherever) and get 1 Megaohm, 1/4 watt resistor, and put that in line with the black wire of the second o2 sensor.

I dont know if this will work on a gst/x, it will work on a gs/rs

Also, do emissions tests involve inspecting the sensors? If a sensor (with resistor) was tucked away, but read out, i assume, constant voltage... would it pass?

I assume it would pass emission, because it is fooling the ECU. However i have heard that some emission inspectors do not even check the cat. And if they did i dont think they would go as far as reading the voltage.
But i dont have emission so i dont know :)

During choke you are in open loop. The ECU waits a bit for the o2 sensor to warm up and start to actually work before relying on it. Until you reach operating temperature though (180 or so IIRC) it will not adjust the trims. You need the front O2 for the car to run well in closed loop. It will default to some (rich) open loop setting if the front o2 isnt there.

You want to monitor the front O2 sensor. The rear signal will be off after the exhust goes through the cat. Though it may not be by much.

I got a problem with my 02 sensor. it says its bank 1 sensor 2. would it be good to get a universal or should i just get the original.

I got a problem with my 02 sensor. it says its bank 1 sensor 2. would it be good to get a universal or should i just get the original.

I would recommend OEM for the O2 sensor. They are about $100 from Satan. Most of the aftermarket sensors do not include the O2 sensor heater. If it doesn't (identifiable by only having 2 wires) it will trigger a CEL. Also, the aftermarket sensors require you to hardwire them by tapping into the factory harness. Just spend a little more money and go OEM. But that's just my :2cents:

do you guys think if i fix the o2 sensor it will eliminate other sensor problems that i have like the coolant temp.

hey if you have dsmlink and can get rid of the CEL is there a way to totally remove the second o2 sensor (2g turbo) so its not hanging from under your car in foil or sitting in your exhaust for no reason. i just removed my stock exhaust and am gettin dsmlink. also what are those metal straps or bands attatched to the stock exhaust?(attatches the shell to exhaust.) at first i thought they were ground wires but they arent groudnign anything. i believe there are 2 of them.