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You probably mean CO which is short for carbon monoxide. The code 25 is for the oxygen sensor. The car is running slightly rich. On these older Corolla's I have found that the oxygen sensor when malfunctioning will affect the CO output. If you change the oxygen sensor it will probably pass the emissions test.
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I am confused in your initial statement you said there was a code 25. This is a malfunction of the O2 sensor. Is the O2 sensor toggling from .1 Volts to .9 volts several times in one minute?
The Haynes manual is very generic and description. A code 25 is an indication of a lean condition. A lean condition will screw up the emissions output at the tailpipe. 90% of the time when you see a malfunction in the O2 sensor and you get a code 25 it will fail the emissions on CO. To properly test an oxygen sensor you need a lab scope. If you monitor the voltage swings from 0 to 1 volt you should see 8 to 10 voltage swings per minute. You can check this with a volt meter also it just is not as accurate. You can probe the OX1 pin in the diagnostic connector under the hood for easy access. More than likely you will find that the oxygen sensor is bad.
Have you actually checked the voltage swings of the OX1 pin in the check connector? HC failures are due to partial burn of the air fuel ratio mixture. If you have already checked and replaced Rotor wires plugs and so on these things are all ruled out. Secondary ignition is the primary reason for failure on HC emissions. I understand your situation being a poor college student and all. But I have my limitations setting behind his keyboard. If the compression is low on the cylinders or if the rings are weak excessive oil burning can cause HC to be too high. The only other cheap option I can think of would be to dump some STP in the oil to try and thicken up a gap in the cylinders. Not exactly my kind of fix but it might work for what it's worth give it a try. I am not a fan of used engine parts at least not without their own testing. The only other option would be the catalytic converter. If the catalyst is not function properly it cannot convert the right chemical mixtures to eliminate the HC and the CO2 and the CO. Unfortunately converters are a last resort and very expensive let me know how things go.
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