What to Know About Texas Vehicle Emission Testing

As the wheels of change turn in Texas's vehicle regulations, it's imperative to grasp the ins and outs of vehicle emission testing in the Lone Star State.

With the recent decision to eliminate Texas State inspections requirements for vehicles, the focus shifts to the vital role of emission testing. But until this legislative change takes effect in 2025, vehicle owners in Texas still have mandatory safety inspections ahead.

Furthermore, emissions testing remains a requirement, depending on the county. In this article, we delve into what every Texan needs to know about vehicle emission testing.

Understanding Texas Vehicle Emissions Testing

The Texas Emissions Test, also known as a smog test, is a regulatory procedure that monitors and controls vehicle emissions to ensure cleaner air quality and less pollution.

The Texas Department of Public Safety (DPS) and the Texas Commission on Environment Quality (TCEQ) initiated vehicle emissions inspections on May 1, 2002, to improve the air quality of 17 designated counties in Texas.

Since March 1, 2015, Texas has implemented a "Two Steps, One Sticker" system, combining vehicle inspection and registration renewal into a single registration sticker. This means that your vehicle must pass a state inspection and potentially require an emissions test before it can be legally registered.

The counties that mandate emissions testing include Brazoria, Collin, Dallas, Denton, El Paso, Ellis, Fort Bend, Galveston, Harris, Johnson, Kaufman, Montgomery, Parker, Rockwall, Tarrant, Travis, and Williamson.

Requirements for Emissions Testing

In Texas, there are specific requirements for emissions testing. Certain vehicles are exempt from this testing, including:

  • New vehicles made within the last 2 model years
  • Vehicles manufactured more than 24 years ago
  • Diesel-powered vehicles, motorcycles, and electric vehicles
  • New cars with the initial two-year registration certificate

Simply put, if your car falls within the range of 2 to 24 years since its manufacture, it must pass emissions testing. All gasoline-powered vehicles are also obligated to undergo emissions inspection.

Note that you cannot register your vehicle if it has not completed the inspection and emission test.

The Process of Texas Vehicle Emission Testing

On-Board Diagnostics - Second Generation (OBD-II) Test

Vehicles manufactured in 1996 and newer undergo an OBD-II inspection, which utilizes a computerized system to monitor emissions-related components and ensure their proper functionality. This advanced test employs an onboard computer to detect exhaust or emission issues in advance, allowing for cost-effective repairs.

During the OBD-II inspection, an OBD-II scanner is used to download stored information from the vehicle's internal computer. This scanner checks the emission system for any malfunctions. If an issue is detected, the engine computer triggers the "Check Engine" light on the dashboard, and a Diagnostic Trouble Code (DTC) is stored in the computer memory. Technicians can then retrieve these codes from the scanner and make necessary repairs.

The Malfunction Indicator Lamp (MIL) or Check Engine Light check is performed during the OBD-II test for 1996 and newer vehicles. These lights should turn on briefly when the ignition is switched on and then turn off when the engine starts. If the light fails to function correctly, it indicates that the OBD-II has detected an issue, and a DTC is set, which leads to a failed emissions test.

Tailpipe Emissions Testing for Older Vehicles

Tailpipe emissions tests, such as the Acceleration Simulation Mode (ASM) and Two-Speed Idle (TSI) tests, were previously used for older vehicles manufactured before 1995 that lacked on-board diagnostic systems. These tests involved inserting a probe into the exhaust pipe to measure engine speed and assessing emissions at idle and 2500 RPM.

According to the Texas Department of Public Safety (DPS), these tests are obsolete as older cars are now exempt from emissions inspection. These vehicles still need to undergo an annual safety inspection for registration requirements.

Understanding Emission Control Devices and Their Function

  1. Catalytic Converter: Converts harmful gases, such as carbon monoxide, nitrogen oxides, and hydrocarbons, into less harmful substances through a chemical reaction.
  2. Exhaust Gas Recirculation (EGR) System: Recirculates exhaust gases back into the engine to lower combustion temperatures and reduce nitrogen oxide emissions.
  3. Oxygen Sensor: Monitors oxygen levels in exhaust gases, allowing for precise air-fuel mixture adjustments to optimize combustion and reduce emissions.
  4. Evaporative Emission Control System (EVAP): Prevents fuel vapors from being released into the atmosphere by capturing and directing them back to the engine for combustion.
  5. Diesel Particulate Filter (DPF): Traps and removes soot and particulate matter from diesel engine exhaust, reducing emissions of harmful particles.

Common Reasons for Failed Emissions Tests and the Importance of Regular Maintenance

Regular maintenance is crucial to ensure proper emission control and overall vehicle performance. Not maintaining your car can result in various issues that can cause it to fail an emissions test. Here are some common reasons for failed emissions tests:

  • Faulty catalytic converter
  • Malfunctioning oxygen sensor
  • Loose or damaged gas cap
  • Fuel metering not meeting required specifications
  • Worn-out spark plugs
  • Inaccurate synchronization of the ignition timing
  • Dysfunctional injection system
  • Improper routing of vacuum lines
  • Defective Positive Crankcase Ventilation (PCV) System
  • "Check Engine Light" remaining on

You must keep up with regular tune-ups, change your oil and air filters, replace worn-out spark plugs, and check for leaks or other issues in your exhaust system. This can keep your vehicle emissions within acceptable limits, ensure regulations compliance, and protect the environment.

Consequences of Failing Emissions Testing and Compliance

Failing the emissions test prevents vehicle registration renewal, rendering the vehicle unfit for legal use on public roads.

If your vehicle fails the emissions test, you must address the required repairs and have it retested before you can proceed with the registration. Repairs could involve replacing parts, fixing exhaust systems, or addressing engine problems. This is not only time-consuming but also incurs financial implications, including repair costs and retesting fees.

Furthermore, failing emissions tests contributes to poor air quality, environmental concerns, smog formation, greenhouse gas emissions, and respiratory health problems. Consequently, enforcing emissions testing and compliance aims to mitigate these adverse effects on the environment and public health.

Acceptable Emission Levels and Compliance

Texas follows the federal regulations set by the Environmental Protection Agency (EPA) regarding vehicle emissions. The emission standards vary based on the type of vehicle and its model year. These regulations aim to control air pollution and promote cleaner air quality across the state.

For light-duty vehicles, trucks, and motorcycles, the EPA has implemented a tiered system of emission standards. These standards specify the maximum allowable emissions for various pollutants, including carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (PM), and non-methane organic gases (NMOG). The standards become more stringent with each subsequent model year.

Steps to Take If a Vehicle Does Not Meet Emission Standards

If a vehicle does not meet emission standards, you must take it to a DPS-certified inspection station, get the necessary repairs, and have it retested.

But what if your vehicle still doesn't pass? Or if the required repairs to make the vehicle compliant are too expensive or impracticable?

The State of Texas provides waivers and extensions to eligible car owners.

Importance of Addressing Emission Issues Promptly

Failing an emissions test indicates that your vehicle releases pollutants beyond acceptable limits, contributing to air pollution. Promptly addressing these issues reduces harmful emissions and improves air quality and public health. It also helps you comply with state regulations, avoiding penalties and restrictions.

Moreover, timely repairs prevent further damage to the vehicle's components and improve its overall performance and fuel efficiency.


Emissions testing in Texas is essential for identifying vehicles that emit excessive pollutants and ensuring they adhere to acceptable emission limits. It promotes cleaner air and reduces environmental impact. Regular vehicle maintenance and compliance results in optimal vehicle performance, improved fuel efficiency, and reduced maintenance costs.

Note that emission testing requirements and regulations can be subject to change, so it's recommended to visit the official Texas Department of Public Safety (DPS), Texas Department of Motor Vehicles (TxDMV), or Texas Commission on Environmental Quality (TCEQ) websites for the most up-to-date information.