67-81 - test every year via sniff. Gas cap test included.
1975+ also looks to see if EGR is present
82-95 - test every 2 years via sniff. Includes Gas cap and EGR underhood inspection
96+ OBDII - test every 2 years plug and play. No codes? You're good.
All HD and Diesel trucks regardless of year 1967+ test every year with a sniff. The diesel test is abusive to your vehicle.
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