Connecticut Emissions Testing: Everything You Need to Know (2025 Guide)

Every two years, Connecticut requires most registered vehicles to pass an emissions test.It's the law — and if your car fails, you can't renew your registration until it passes.Here's everything you need to know about CT emissions testing in 2025.

Who Needs an Emissions Test ?

Connecticut requires emissions testing for most gasoline - powered vehicles that are model year 1996 or newer and registered in the state.Your vehicle needs testing every two years . You'll receive a notice from the DMV when your test is due.

Exemptions

• New vehicles: Cars less than 4 model years old are exempt from testing
• Diesel vehicles: Currently exempt from the standard OBD-II test
• Electric and hybrid vehicles: Battery-electric vehicles are exempt; hybrids still require testing
• Classic / antique vehicles: Vehicles 25+ years old with antique plates are exempt

What the Test Actually Checks

Connecticut uses the OBD - II(On - Board Diagnostics) test. A technician plugs a scanner into your car's diagnostic port (usually under the dashboard, driver's side) and reads the data your car's computer stores. The scanner checks for:

• Active trouble codes: Any stored "check engine light" codes will cause an automatic failure
• Readiness monitors: Your car's computer runs self-tests on emissions-related systems. If too many monitors are "not ready" (incomplete), the test will be rejected — not failed, but you'll need to drive more and come back
• Visual inspection: The technician checks that your gas cap seals properly and that no emissions equipment has been tampered with or removed

Top 5 Reasons Cars Fail Emissions in CT

1. Check Engine Light Is On

This is the number one reason for failure.If your check engine light is illuminated, your car will automatically fail — even if everything else checks out. Get the codes read before going to the test station.

2. Catalytic Converter Problems

The catalytic converter cleans exhaust emissions.When it fails, it throws a P0420 or P0430 code.Replacement costs $800–$2, 500 depending on the vehicle. Aftermarket converters are legal in Connecticut but must be CARB-compliant.

3. Oxygen Sensor Failure

O2 sensors monitor how efficiently your engine burns fuel.A failed sensor triggers emissions codes and can reduce your fuel economy by 10–40 %.Replacement typically costs $150–$400 .

4. EVAP System Leaks

The evaporative emission control(EVAP) system prevents fuel vapors from escaping into the atmosphere.A loose, cracked, or missing gas cap is the most common EVAP issue.Replacing the gas cap costs $15–$30 and fixes the problem about 30% of the time.

5. Incomplete Readiness Monitors

If you recently disconnected your battery, had codes cleared, or had major repairs, your car's computer may not have completed its self-tests. You'll need to drive 50–100 miles through a mix of city and highway driving to allow the monitors to complete.

How to Prepare for Your Emissions Test

1. Check your gas cap: Make sure it clicks when you tighten it. Replace it if the rubber seal is cracked.
2. Drive for at least 50 miles: If your battery was recently disconnected or codes were cleared, drive the car for a few days before testing.
3. Get the check engine light diagnosed first: If the light is on, bring it to a mechanic before going to the testing station. Don't waste your time and $20 on a test you'll fail.
4. Warm up the engine: Drive to the test station rather than having the car towed. A warm engine produces cleaner emissions readings.
5. Make sure your car is well - maintained: Regular oil changes, fresh spark plugs, and clean air filters all help your engine burn fuel cleanly.

What If You Fail ?

If your car fails, you have 60 days to get it repaired and retested. The retest is free if done within this window. If repairs exceed a cost threshold (currently around $450–$550 for most model years), you may qualify for a waiver — which means Connecticut will let you register the car even though it didn't fully pass, as long as you've made a good - faith effort to repair it.

Emissions Test Cost

The test itself costs approximately $20 at authorized Connecticut emissions testing stations.

We Fix Emissions Problems

At Strong's Alignment Services, we diagnose and repair the issues that cause emission test failures — from oxygen sensors and catalytic converters to EVAP leaks and engine tune-ups. If your check engine light is on, bring it in before your test and save yourself a trip.

Call: (860) 569 - 1440 | 195 Silver Lane, East Hartford, CT