Back to all

Why did my vehicle fail its emissions test?

July 21, 2025

We all want to breathe clean air and keep our planet green.  One of the ways to do that is by requiring all vehicles with internal combustion engines to pass an emissions test.  That’s because these vehicles can be big polluters.  

Everyone wants their car to pass the test so they can continue their day-to-day driving routine.  But then comes that one day when they tell you your vehicle has failed.  Why does that happen?

One of the most common reasons is that your gas cap is leaking or loose (or maybe you don’t even have one on your vehicle!).  The gas cap is supposed to seal in vapors, but sometimes its seals or gaskets dry out or crack. Replace it, and you’re good to go.

Another common reason your vehicle failed the test is that your catalytic converter isn’t doing its job. It is supposed to convert toxic gases from your engine into ones that don’t pollute the air.  There are many reasons the catalytic converter may not be working.  It may be physically damaged, or the steady flow of exhaust gases has fouled the converter. 

You may have a bad oxygen sensor. It’s supposed to measure how much oxygen is in your exhaust.  This can cause the air-fuel mixture to be too rich, resulting in too many hydrocarbons in the exhaust. Your ignition system may also have problems, such as worn-out spark plugs.  Dirty fuel injectors can contribute to excess emissions.

Believe it or not, a dirty engine air filter may be the culprit. If it hasn’t been replaced for a while, your vehicle may be emitting too many hydrocarbons -  something that will cause your vehicle to fail the emissions test.

You can usually avoid a failed emissions test by maintaining your vehicle regularly.  The best way to do that is by bringing it to our trained technicians, so your vehicle gets the service it needs, scheduled and non-scheduled.  Plus, you’ll be doing your part to keep our air clean.

Tuffy Tire & Auto Service Center Waukesha
S31 W24721 Sunset Drive
Waukesha, Wisconsin 53189
262-513-9500

More articles from Tuffy Waukesha

Plugging Away (Spark Plug Replacement)

September 14, 2025

If you drive a vehicle with a gasoline engine, thank the spark plugs for igniting the gasoline/air mixture in each cylinder that keeps your vehicle traveling smoothly down the road every day. Your spark plugs work hard, firing thousands of times in a well-choreographed order, and you usually don... More

Let's Clear Some Things Up (Headlight Restoration)

September 8, 2025

You know how exposing your skin to sunlight can cause sunburn and other unhealthy things. Sunlight can also create major problems for your headlights. After they've been exposed to ultraviolet light, acrylic headlights can yellow and fog due to oxidation. And when that happens, less light can ... More

Why Alignment Matters (Vehicle Alignment Maintenance)

September 1, 2025

It's one of those things that can happen on a multi-day road adventure or a quick trip to the grocery store. You hit a pothole, go too fast over a speed bump, nail a curb. When it happens, you think, "I wonder if that was hard enough to do any damage?" But things seem ok and you continue on yo... More