How can it be that CO is low but both HC and NOx are high?
81 VW Scirocco with 255K mi. Since last check, car only driven 3300 mi short trips.1st check: Initially low CO and NOx but high HC.
Changed oil from 20W/50 GTX to Mobil 1 10W/40 High Mileage. New spark plugs. Drove about 150 miles with hopes of cleaning carbon.
2nd check: Both HC and NOx high, with HC higher than before.Gave the car to a mechanic who previously had gotten the car to pass. Could not find anything out-of-spec (but he has no exhaust gas analyzer).
3rd check: HC and NOx gone up into 'gross polluter' numbers.Mechanic and smog checker now recommend new CAT and O2 sensor.
Comments?
Thanks, nf