Oxygen sensor? '90 Celica

I noticed a few weeks ago that while driving at about 55mph. for a few minutes, my check engine light will come on. When I let go of the gas
pedal, the check engine light will shut off after a few seconds and will usually remain off for the rest of the night. After reading through the archives within this group, I came to the conclusion that it could be my oxygen sensor. I have also noticed that my gas mileage has dramatically decreased, and from what I also read, is a sign of a failing oxygen sensor.
I went to have it replaced earlier today and was told that there were TWO oxygen sensors for my vehicle. I was not able to locate a second one (there is one before the catalytic converter and according to the shop I took my vehicle to, there's one after). Is there one or two oxygen sensors for my vehicle?
Also, if I had the oxygen sensor replaced, would that likely cure the problem with my intermittent check engine light and decreasing gas mileage?
Thanks!!
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The easiest way to tell if there is a second oxygen sensor in your car is to look under the car. If you see something shaped like a cigar with a wire attached sticking out of the exhaust pipe after the catalytic converter, then it has 2 sensors. All vehicles equipped with the OBD II diagnostic system have O2 sensors before and after the catalytic converter, but your vehicle is not OBD II.
As far as whether replacing an oxygen sensor is likely to cure the intermittent check engine light and decreasing fuel mileage, it depends on what is making the check engine light illuminate. You can greatly increase the odds that you will check and repair the proper component if you pull the trouble codes that are likely stored in your car's computer.
Look under the driver's side of the dashboard for a covered connector that is marked "diag" or "diagnostic." Open the cover, and with a straightened-out paper clip, connect terminals TE1 and E1. The terminals will be marked on the connector itself or on the inside of the cover. Turn the ignition switch on and count the flashes of the check engine light, noting the short and long pauses between flashes until the pattern starts to repeat itself. For example, with X representing a flash, XXX (short pause) X means 41. There may be more than 1 code stored, and each code has 2 digits. Let us know what the codes are, and hopefully, someone will have the trouble codes for your car so you will have a better idea of what to look for.
If you are willing to invest in an inexpensive volt/ohm meter (under $25 at Radio Shack) then you can check your O2 sensor output.
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Ray O
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Thank you for your quick response Ray.
With the engine off, the code was <b>21</b>.
I turned the engine on and received the same code. When I revved the engine to 2500rpm, I received an additional code of <b>51</b>.
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Thank you for your quick response Ray.
With the engine off, the code was 21.
I turned the engine on and received the same code. When I revved the engine to 2500rpm, I received an additional code of 51. However, when I stopped applying the gas pedal, the only code that remained was code 21.
According to my Chilton's manual, code 21 is "OXYGEN SENSOR SIGNAL." Code 51 is "SWITCH CONDITION SIGNAL."
Now, also according to Chilton's, it says this could be a "short" in the oxygen sensor. Would simply replacing the oxygen sensor be a safe bet to resolving this issue or could it be something more serious?
Thanks again!
Ray O wrote:

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Assuming that the Chilton's list of codes is correct, then there is a pretty good chance that replacing the oxygen sensor in the exhaust manifold will cure the problem. You can confirm this by checking the voltage coming from the O2 sensor, the Chilton's manual should explain how to do this, or you can gamble and just replace the sensor.
You can get aftermarket O2 sensors for a lot less than an OEM sensor, but from what I have read, aftermarket O2 sensors seem to have a much shorter life. Also, aftermarket sensors often require splicing wires. If you decide to go with an aftermarket sensor, cut the pigtail off of the old O2 sensor and splice the aftermarket sensor to the pigtail so you maintain the factory connector. The best way to splice the wire is to twist and solder the connection and then seal with heat shrink tubing. Crimp connections can fail and cause check engine lights.
You can clear the codes stored in the car's computer by removing the EFI/main fuse for 60 seconds.
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Thank you for your quick response Ray.
With the engine off, the code was 21.
I turned the engine on and received the same code. When I revved the engine to 2500rpm, I received an additional code of 51. However, when I stopped applying the gas pedal, the only code that remained was code 21.
According to my Chilton's manual, code 21 is "OXYGEN SENSOR SIGNAL." Code 51 is "SWITCH CONDITION SIGNAL."
Now, also according to Chilton's, it says this could be a "short" in the oxygen sensor or the oxygen sensor heater. Would simply replacing the oxygen sensor be a safe bet to resolving this issue or could it be something more serious?
Thanks again!
Ray O wrote:

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The oxygen sensor is the most often changed good part on the modern automobile. Have the system analyzed, to determine WHY the sensor is being contaminated, would be a better solution to your problem. If could be just a bad spark plug
mike

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