Vehicle Variability

My vehicle failed its first emissions test. It passed a second test without getting any repairs. What gives? Is the testing equipment faulty? Are the test drivers to blame?

This situation is of concern to motorists, to the State of Colorado and to Envirotest Systems. Variable test results occasionally occur.

Test Lane Consistency

The emissions testing equipment is standardized in all of the I/M 240 emissions testing lanes. Each test lane self-checks every two hours. If this check reveals a problem with lane calibrations, the lane automatically shuts down, preventing inaccurate testing. In addition, computer software controls the dynamometer testing. The test driver cannot exert more than a marginal influence on the test results.

Vehicles are Variable

Variable test results are generally caused by the vehicle itself, and can happen for a number of reasons. The catalytic converter may not have been adequately preconditioned, or warmed up, for the first test. Driving a vehicle for at least 15 minutes or so to bring it up to operating temperature before taking it in for an emissions test is recommended, as cold vehicles emit a higher level of pollution. The vehicle may have switches and sensors that may be causing problems intermittently. This can be a particular problem with computer-controlled vehicles, and certainly can be a frustrating problem for repair technicians to diagnose.

Small differences in test results from test to test are normal for vehicles, particularly those that are computer-controlled. But if the vehicle is close to the pollution limits to begin with, a small difference in the test result can make the difference in failing or passing the emissions test.

The I/M 240 dynamometer test is very comprehensive. The vehicle is placed on the dynamometer, and the pollution emitted from the vehicle as it simulates driving down the road, accelerating, decelerating, and cruising is collected and analyzed. The idle test detects the pollution emitted only while a vehicle idles. Therefore, the I/M 240 test is much more likely than the idle test to detect an intermittent problem.

Intermittent Problems Can be Pain to Diagnose

Solving these intermittent vehicle problems sometimes requires real detective work by repair technicians. This is one of the reasons vehicle repair has become a much more high-tech business than it formerly was, and why continuing technician training is key to a repair shop's success. It is also why a diagnostic fee is a good investment.

The state is concerned about vehicle variability because a vehicle that manages to pass the emissions test but still goes unrepaired continues to pollute significantly more than it should. The Colorado Air Pollution Control Division maintains several Emissions Technical Centers around the Denver-Boulder area for the purpose of helping motorists diagnose difficult vehicle emissions problems. They may be contacted at 303-744-2442 (and press 3) for information or an appointment.

How do I find a reliable repair technician?
How to I find a repair shop that has a good track record of fixing emissions repairs?