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.
RepairPal Certified shops can diagnose and repair your 1997 Chevrolet Monte Carlo. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.