Understanding the Three Types of Vehicle Emission Testing

The three primary types of vehicle emission testing are: On-Board Diagnostics (OBD) inspections, tailpipe exhaust measurements (idle or dynamometer-based), and evaporative emissions (EVAP) system tests. In practice, jurisdictions combine these methods based on vehicle age, fuel type, and local air-quality goals to verify that cars meet pollution standards and to identify malfunctioning emission-control systems.

Why Emissions Testing Exists

Vehicle emissions testing is designed to reduce harmful pollutants—such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM)—that contribute to smog and health problems. Modern programs prioritize quick, reliable tests that reflect real-world performance, relying heavily on a vehicle’s built-in diagnostics for newer models while still using direct exhaust sampling or system integrity checks when appropriate.

The Three Types of Vehicle Emission Testing

1) On-Board Diagnostics (OBD) Inspection

OBD testing electronically interrogates a vehicle’s computer to check for stored trouble codes, the status of emissions-related monitors (readiness), and whether the malfunction indicator lamp (MIL/check engine light) is commanded on. For most light-duty vehicles from model year 1996 onward (and later phases for some diesel applications), OBD-II is the primary inspection method because it continuously monitors critical systems like the catalytic converter, oxygen sensors, EGR, misfire, and EVAP leak detection during normal driving. If the MIL is on or required monitors are “not ready,” the vehicle typically fails until repairs are completed and monitors complete successfully.

2) Tailpipe Exhaust Measurement

Tailpipe testing directly measures pollutants at the exhaust. It’s commonly used for older vehicles without robust OBD capability, for certain diesel checks, or as a follow-up where direct sampling is required. Depending on local rules, tests may be performed under load on a dynamometer, at a steady elevated speed, or at idle. The analysis targets HC, CO, and NOx in spark-ignition engines; for diesels, opacity (smoke density) or specific emissions may be assessed.

The following list outlines the most common tailpipe test methods and what they measure or simulate.

• Acceleration Simulation Mode (ASM): A loaded-mode dynamometer test (e.g., ASM5015/2525) that simulates urban driving to measure HC, CO, and NOx under controlled loads and speeds.
• IM240 (or similar drive-cycle tests): A 240-second transient cycle on a dynamometer that mimics real driving, capturing emissions during acceleration, cruise, and deceleration.
• Two-Speed Idle (TSI): A simpler idle and high-idle (around 2,500 rpm) test measuring HC and CO when load testing isn’t feasible.
• Diesel opacity/snap-acceleration: An opacity meter assesses smoke density during rapid throttle changes, used primarily for older diesel vehicles without advanced OBD or aftertreatment sensors.

While nomenclature varies by region, these methods share the goal of quantifying pollutants at the tailpipe under consistent conditions to compare against standards.

3) Evaporative Emissions (EVAP) System Tests

EVAP testing checks for fuel vapor leaks that would otherwise escape from the tank, lines, and charcoal canister. Historically, inspection programs used a gas-cap pressure test or applied pressure/flow to the EVAP system. Today, many jurisdictions rely on the vehicle’s OBD EVAP monitors (which test for small leaks—often on the order of 0.040 inch—during normal driving). Standalone gas-cap tests are increasingly limited to older vehicles that lack reliable OBD EVAP diagnostics.

How Programs Choose Among Them

Testing protocols vary by jurisdiction and vehicle population. Newer gasoline vehicles typically receive OBD-only inspections because they provide comprehensive, self-monitoring coverage with minimal test time. Older gasoline vehicles often require a tailpipe test, while EVAP checks may be OBD-based or supplemented by a gas-cap or system integrity test. Diesel light-duty vehicles may be inspected via OBD plus opacity or visual checks, with heavy-duty diesels sometimes subject to separate roadside or periodic smoke/NOx assessments.

What Different Vehicles Usually Get

The mapping below summarizes which test type is commonly applied, recognizing that local rules can differ and may change over time.

• 1996-and-newer gasoline cars/light trucks: Primarily OBD inspection; tailpipe testing generally not required unless there’s an exception or special program rule.
• Pre-1996 gasoline vehicles: Tailpipe exhaust measurement (TSI, ASM, or IM240) and, in some areas, a gas-cap or EVAP integrity check.
• Light-duty diesel vehicles: OBD (where applicable) and, in many regions, an opacity check; newer models may rely more on OBD for aftertreatment monitoring.
• Special cases (kit cars, gray-market, or vehicles with modified emissions systems): May face enhanced inspections, visual equipment checks, or dynamometer testing.

This framework helps inspectors target the most effective and reliable method for each vehicle class while balancing accuracy, cost, and test time.

Preparing for an Emissions Test

A few practical steps can reduce the risk of an avoidable failure and speed up your inspection.

• Resolve any check-engine light issues and clear pending DTCs with proper repairs, not just code resets.
• Complete a normal drive cycle after repairs so readiness monitors can set; mixed city/highway driving over several trips often works.
• Ensure the fuel cap seals properly and that the EVAP purge/vent lines are intact.
• Verify there are no obvious exhaust leaks, missing catalytic converters, or disconnected sensors.
• For diesels, maintain filters and aftertreatment systems (DPF/SCR) and use the specified fluid (e.g., DEF) where required.

These steps align the vehicle’s self-monitoring with the inspection and reduce the chance of failing for preventable reasons like “not ready” monitors or simple leaks.

Key Takeaways

In summary, emissions programs use three core testing types: OBD inspections to read a vehicle’s self-diagnostics, tailpipe exhaust measurements to directly quantify pollutants, and EVAP system tests to catch fuel-vapor leaks. The specific mix depends on vehicle age, technology, and local regulations, with modern programs increasingly centered on OBD for newer cars and targeted tailpipe or EVAP checks where needed.

What are the types of car emissions?

Cars and trucks that combust fuel also emit smog forming emissions, such as nitrogen oxide, non-methane organic gases, carbon monoxide, particulate matter, and formaldehyde.

Does TN still require emissions testing?

No, Tennessee does not currently have statewide vehicle emissions testing, as the program was discontinued in 2022 for metropolitan counties after the state received approval from the EPA to remove it from its air quality plan. This means residents of major counties, including Davidson, Hamilton, Rutherford, Sumner, Williamson, and Wilson, are no longer required to have their vehicles tested for emissions for registration purposes. 
Why the program ended:

• The Environmental Protection Agency (EPA) approved Tennessee’s air quality plan, which removed the requirement for emissions testing. 
• The removal of the program aimed to ease requirements for drivers and the city. 

Potential future changes: 

• State law allows for an emissions program to be reinstituted if a county becomes noncompliant with EPA air quality standards.

What are the three types of vehicle inspections?

There are three types of vehicle inspections – a courtesy inspection, an insurance inspection, and a 12-point inspection. Courtesy Inspection – a visual inspection of common components (such as lights, oil, and wipers). This is typically done during an oil change or tire changeover.

What are the three general types of emission controls?

The three general types of emission controls are Exhaust, Evaporative, and Crankcase Emission Controls. Exhaust Emission Controls, like catalytic converters and Diesel Particulate Filters, reduce harmful gases in vehicle exhaust.