Why Diesel Vehicles Fail Emissions Tests in Colorado (And What You Can Do About It)
If you’ve ever sat through a diesel emissions test only to end up with a “FAIL” slip, you know how frustrating it can be. You’ve got things to do, places to be—and now you’ve got to figure out what went wrong and how to fix it. At Hinsdale Automotive in Littleton, we see it all the time. The good news? Most diesel emission failures in Colorado come down to a handful of common issues.
Our shop uses a DYNO machine to perform emissions tests for diesel vehicles, which gives us a good look at what’s going on under the hood and under the tailpipe. Whether you’re driving a work truck, a personal diesel SUV, or something in between, here are the most common reasons diesel vehicles fail—and what you can do to prevent them.
1. Worn or Clogged Diesel Particulate Filter (DPF)
The diesel particulate filter is one of the most important parts of your emissions system. Its job is to trap soot before it leaves your exhaust. Over time, that filter fills up. If it isn’t regenerating properly—or if it’s just too full—it can trigger a failure during the emissions test.
Sometimes a DPF can be cleaned and reset. Other times, especially if it’s ignored too long, it needs to be replaced. Either way, if your filter is clogged, your emissions numbers will show it.
2. Bad or Dirty EGR Valve
The Exhaust Gas Recirculation (EGR) system helps control emissions by cycling some exhaust back into the engine to lower combustion temperatures. But diesel exhaust isn’t exactly clean, and over time, that valve can get sticky or clogged with carbon buildup.
When the EGR valve stops working right, your engine burns dirtier and throws off higher NOx levels. We’ve had trucks come in with failed tests where a simple EGR cleaning or replacement solved the issue.
3. Check Engine Light Is On
This one might seem obvious, but it still catches people off guard. If your check engine light is on when you go in for a test, it’s an automatic fail—even if the truck seems to run fine.
A lot of times, it’s something minor. Maybe a faulty sensor or a loose gas cap. But Colorado emissions testers won’t pass a diesel vehicle until that light is off and the system resets. Don’t ignore it—get the code read and see what’s going on before your test.
4. Faulty Sensors or Missing Equipment
Modern diesel engines rely on a network of sensors to control emissions. Oxygen sensors, MAP sensors, and temperature sensors all play a part. If one of them isn’t working, your vehicle may run rich or burn inefficiently. That leads to higher emissions and a failed test.
Sometimes it’s just an old sensor that’s gone bad. Other times, the emissions equipment is physically missing, especially if someone removed the EGR or DPF in the past to boost performance. Either way, the DYNO test will catch it.
5. Excessive Smoke or Opacity
One of the most visual reasons for failure is excessive smoke. If your vehicle smokes under load, even just for a few seconds, the DYNO machine will measure it, and if it’s too much, you won’t pass.
We’ve seen this with older trucks, especially ones that haven’t been tuned or serviced regularly. Sometimes it’s injectors, sometimes it’s poor combustion, and sometimes it’s just neglect. Either way, if your truck is blowing smoke, it needs attention before it passes.
What You Can Do
If your diesel vehicle fails an emissions test, don’t panic. Most of the time, it’s fixable, and many repairs are more straightforward than you might think. The key is knowing what the problem is and working with a shop that understands diesel systems and emissions standards.
At Hinsdale Automotive, we run diesel emissions tests on-site using our DYNO machine and perform the repairs needed to get your vehicle back on the road legally. We know the testing process inside and out, and we’ll walk you through your options if something fails.
If you’re due for a test or just failed one, give us a call. We’re here in Littleton, serving diesel drivers across the Denver Metro area. Whether you’re hauling tools, towing gear, or just driving your daily truck, we’ll help you stay compliant and running clean.
Need help after a failed test? Hinsdale Automotive can diagnose, repair, and retest your diesel vehicle, so you can get back to driving without the headache.