Bill,

actually they are supposed to put a sensor on the tailpipe to measure CO and other emissions coming from the pipe. It is the exact same test as they use on the commercial vehicles for the "clean air" testing. You are using biodiesel, so you are running a lot cleaner then people running dino-diesel. If your engine is in tune and all your parts are working and in adjustment you should pass no problem. Here is how the test is done right from the drive clean site
At an accredited Drive Clean facility, an emissions inspector will drive your car or light-duty truck onto a dynamometer. This machine is like a treadmill that allows the vehicle to be "driven" on one spot at different speeds.

A probe is placed in the tailpipe of your vehicle. The probe is connected to a gas analyzer and computer, which can measure the concentration of nitrogen oxides (NOx) and hydrocarbons (HC), which includes volatile organic compounds, and carbon monoxide (CO).

Your vehicle is accelerated to a steady speed of 40 km per hour. Several emissions readings are taken and then averaged by the computer during this driving simulation. The simulated driving conditions result in measurements of NOx, HC and CO. Then, when the vehicle is returned to idle, the emissions are measured again, in what is called a ôcurb idleö test. The idle portion of the test measures HC and CO.

The test results will be compared by a Drive Clean computer to emissions standards for the vehicle and its model year. For example, if you drive a 1990 model car, it will be tested against 1990 emissions standards with an allowance for vehicle deterioration.

As part of the test, the emissions test inspector also checks the upper portion of the fuel filler pipe for damage or corrosion, to determine if fuel vapours can escape, and pressure tests the gas cap. In addition, if the Malfunction Indicator Light (MIL) on the dashboard is illuminated in 1998 and newer model year vehicles, the owners will be advised that it may indicate a malfunction of the vehicle emissions system.

If your vehicle has a problem that would prevent it from being tested safely, or releases visible smoke from the tailpipe it will not be tested until it has been repaired. Such problems could lead to injury, and/or malfunctioning of the test equipment.

Some vehicles, such as those with full-time all-wheel drive, cannot be tested on a dynamometer. These vehicles receive a two-speed idle test, where emissions of CO and HC are measured at curb idle and 2500 RPM.