New Hampshire requires all automobiles and light-duty trucks less than 20 years old to have an OBD emission system inspection.
Diesel Vehicles:
Model year 1997 and newer diesel vehicles with a GVWR of 8,500 lbs or less must undergo emissions testing.
Exemptions:
1. Vehicles with a GVWR of more than 8,500 lbs
2. Vehicles 20 model years old or older
For more information visit: https://www.nh.gov/safety/divisions/dmv/index-original.htm
New Hampshire Revised Statutes
266:59 Muffler; Prevention of Noise.
I. Every motor vehicle shall at all times be equipped with a muffler as defined in RSA 259:66, in good working order and in constant operation to prevent excessive or unusual noise, and no person shall use a muffler cutout, bypass or similar device upon a motor vehicle on a way.
II. The engine and power mechanism of every motor vehicle shall be so equipped and adjusted as to prevent the escape of excessive fumes, smoke, flame, gas, oil or fuel residue.
III. [Repealed.]
IV. No person shall operate a motor vehicle with a straight pipe exhaust system; "straight pipe exhaust system" means any straight-through exhaust system that does not contain baffles or otherwise does not meet the definition of muffler in RSA 259:66. Any person who violates the provisions of this paragraph shall be guilty of a violation and shall be fined not less than $100 for a first offense, not less than $250 for a second offense in a calendar year, and not less than $500 for a third or subsequent offense in a calendar year.
Antique
Available for vehicles that are more than 25 years old and maintained for use in exhibitions, club activities, parades and other functions of public interest.
Street Rod
Available for vehicles whose body and frame were manufactured before 1949, and replicas thereof, that have been modified for safe road use, and which have been certified by the New Hampshire State Police.
YOM
Vehicles registered as "Antique" and manufactured in 1975 or earlier may display YOM plates with DMV approval. The antique plates and registration must be carried within the vehicle at all times.