How Long Does an Emissions Test Take?

Quick Answer

15–30 minutes is the total time for an emissions test, including check-in, the test itself, and receiving your results. The actual diagnostic scan (OBD-II) takes under 5 minutes — most of your time is spent waiting in line. Drive-through testing stations in some states can complete the process in as little as 10 minutes during off-peak hours.

OBD-II vs. Tailpipe Testing

Test Type	Vehicles	What's Measured	Time
OBD-II scan	1996 and newer	Reads onboard computer for fault codes	2–5 minutes
Tailpipe (ASM/IM240)	Older vehicles, some states	Measures exhaust gases directly	10–15 minutes
Visual inspection	All ages (where required)	Checks catalytic converter, gas cap, EGR	5–10 minutes
Diesel opacity	Diesel vehicles	Measures smoke density	5–10 minutes

Most states have transitioned to OBD-II testing for vehicles 1996 and newer, which is faster and more comprehensive than tailpipe testing.

What the Test Checks

An OBD-II emissions test reads your vehicle's onboard diagnostic system for:

• Check engine light status — if the light is on, you automatically fail.
• Readiness monitors — internal self-tests the computer runs on emission systems. Most states require a certain number to be "ready" (typically all or all minus one).
• Diagnostic trouble codes (DTCs) — stored codes related to emissions components.
• Catalytic converter efficiency — the computer monitors converter performance.
• Oxygen sensor function — ensures proper fuel-air mixture monitoring.
• Evaporative system (EVAP) — checks for fuel vapor leaks.

Common Reasons for Failure

Reason	Frequency	Fix Difficulty	Typical Cost
Check engine light on	Very common	Varies	$100–$1,000+
Readiness monitors not set	Common	Drive 50–100 miles	Free (just drive)
Faulty oxygen sensor	Common	Moderate	$150–$300
Catalytic converter failure	Moderate	Difficult	$500–$2,500
Gas cap loose or failing	Common	Easy	$10–$25
EVAP system leak	Moderate	Moderate	$150–$500
EGR valve malfunction	Less common	Moderate	$200–$400

How to Prepare Your Vehicle

• Clear the check engine light issue, not just the code. Clearing codes with a scanner resets all readiness monitors, and you will fail for "not ready."
• Drive at least 50–100 miles after any battery disconnect or code clear so readiness monitors complete their cycles.
• Check your gas cap — tighten it until it clicks. A loose cap triggers the EVAP monitor and can cause a fail.
• Get an oil change if overdue — dirty oil increases hydrocarbon emissions.
• Drive at highway speed for 20 minutes before the test — a warm engine and catalytic converter perform better.
• Make sure tires are properly inflated — some states include a visual or safety inspection alongside emissions.

State Requirements

Not all states require emissions testing. As of 2025, approximately 31 states and the District of Columbia have some form of emissions inspection program. Requirements vary significantly:

• Annual testing: Most states with programs (e.g., Texas, Virginia, Illinois)
• Biennial testing: Some states test every two years (e.g., parts of New York)
• County-based: Many states only test in metro areas (e.g., Colorado, Arizona, Georgia)
• New vehicle exemptions: Most states exempt vehicles under 2–4 years old
• Classic vehicle exemptions: Vehicles over 25 years old are often exempt

Check your state's DMV website for specific requirements.

If You Fail: Retest Timeline

• Fix the issue first — retesting without a repair is a waste of time and money.
• Free retest window: Most states offer a free retest within 15–30 days of failure.
• Drive cycles after repair: After fixing the issue, drive 50–100 miles over several days so readiness monitors reset.
• Waivers: Many states offer repair cost waivers — if you spend a minimum amount on repairs (typically $150–$800) and still cannot pass, you may receive a waiver to register your vehicle.
• Typical retest: Same 15–30 minutes once monitors are ready.