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emmission codes on 1997 Chevrolet Monte Carlo

do u know what the code po121 means for 97 monte carlo 3.1 v6? or po420 on 98 merury sable?

Asked by for the 1997 Chevrolet Monte Carlo
The p0121 on the monte carlo is a throttle position fault. More testing would be needed to determine if the sensor is actually bad. The p0420 on the mercury is a catalytic converter fault. Most likely indicating the converter should be replaced.