Light-duty diesel testing: dynamometer and opacity

Diesel rules are different from the gasoline rules in three ways: the weight line sits at 14,000 pounds, the test has three parts, and heavy trucks are checked by enforcement officers instead of at a station. This guide explains the rules as the Nevada DMV states them and the terms behind them. For any diesel, call the service center before you bring it.

At a glance

• A diesel needs an annual test if its GVWR is 14,000 pounds or less, it is based in urban Clark or Washoe County, and it is model year 1968 or newer.
• The light-duty diesel inspection is a dynamometer test, an opacity test and a visual inspection of emission components.
• Diesels over 14,000 pounds GVWR skip the annual test but face statewide roadside enforcement.
• Call the service center before you bring a diesel.

Which diesels need the annual test

The Nevada DMV's diesel page lists three conditions that must all be true. The vehicle has a manufacturer's Gross Vehicle Weight Rating of 14,000 pounds or less. It is based in the urban areas of Clark or Washoe County. And it is model year 1968 or newer, with new vehicles exempt for their first three registrations and tested at the fourth.

The weight that counts is the manufacturer's rating, which the DMV notes may differ from the declared weight used for registration fees. The simplest check is the renewal notice, which says whether a smog check is required. If you are still not sure, the DMV tells you to contact the Reno or Sahara emission lab, and the Reno lab number is (775) 684-3580.

The three parts of the test

The DMV describes the light-duty diesel inspection as comprehensive, with three pieces: a dynamometer test, an opacity test measuring exhaust density, and a visual inspection of emission components. It is not the plug-in OBD II test that most gasoline vehicles get.

The dynamometer is a set of rollers that lets the vehicle be tested while it is held in place. Opacity is how much light the exhaust smoke blocks, which the DMV calls exhaust density. The visual inspection looks at the emission components on the vehicle itself.

All-wheel drive and traction control

Because the vehicle sits on a dynamometer, drivetrain matters. The DMV says some stations can test only two-wheel-drive vehicles or four-wheel-drive vehicles running in two-wheel mode, and that the capability to test full-time all-wheel drive with traction control is limited by station equipment and by wheelbase. If your diesel is all-wheel drive, ask before you drive over, and the DMV's online emission station search is the official place to look up stations.

Call before you bring a diesel

Diesel details depend on the specific vehicle, so the instruction is simple: call the service center before you bring a diesel. Reno Northwest is at (775) 787-1000 and Reno South is at (775) 828-6996. The same call is the right move for a motorhome or a large truck. Our diesel trucks page and motorhome and heavy truck page collect the common questions.

Diesels over 14,000 pounds

A diesel with a GVWR over 14,000 pounds is not required to participate in the annual smog check program. It is instead subject to statewide roadside enforcement by the DMV Heavy Duty Diesel Enforcement Team, which the DMV describes as sworn peace officers. They conduct traffic stops for observed violations and periodic inspections at highway check sites and weigh stations.

The standard is the SAE J1667 snap acceleration test for exhaust opacity. The DMV states that these standards apply to all vehicles traveling in Nevada, including those registered in other states.

Waivers for diesels

The DMV's diesel page points to a separate document, form EC 004, for the waiver requirements that apply to light-duty diesels that still fail after repairs. The general rules still hold: a failed test starts the process, money spent before the first failed test does not count, and a vehicle that smokes cannot be waived. Read the EC 004 form or call the Reno emission lab before you spend money on a repair.

Validity and timing

As with gasoline vehicles, a standard diesel result is valid for 90 days, and 180 days if the vehicle was bought from a Nevada dealer. Testing early leaves time for repairs before the registration deadline, which matters more for a diesel because its repairs can take longer to arrange.