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Question My son has an 81 Toyota 4x4 with a 22R engine in Berkely. Went to pass the smog check; he is just a little high on CO (I believehigh RPM). Several years ago we replaced the oxygen sensor andnow the mechanic claims it is not working. He says it has aconstant voltage. Am I correct in understanding that a properlyworking O2 sensor generates voltage pulses? Is it likely thatif the voltage is constant, that replacing the unit will lowerthe CO to an acceptable level? Thank you.

Reply An oxygen sensor generates a voltage signal that varies between 100 and 900 millivolts depending upon the fuel mixture.If the fuel mixture is fixed--either rich or lean--the oxygen sensor voltage will remain fixed.If the mixture is toggling from rich to lean, the oxygen sensor voltage should vary accordingly.The oxygen sensor can be tested with a scope or digital volt meter:low voltage with a lean mixture, high voltage with a rich mixture. Be sure a diagnostic check is performed on the vehicle's fuel control and emission control systems.Other items such as air filter, fuel-fouled oil, and need for minor adjustments are common causes for seal failure on that vehicle. Angelo Silverado Auto Service

Followup Thank you for taking the time to reply to my question. Previous advice I received regarding the operation of the oxygen sensor was clearly incorrect. John