If your vehicle was built in an odd-numbered year between 1985 and 2001 -- and you live in southern or eastern Ontario -- get ready to have a probe shoved up your tailpipe.

Since 1999, Ontario's Drive Clean testing program has required owners of private vehicles that are three to 19 years old to undergo a $35 emissions test every two years.

Despite concerns about its effectiveness, and talk last week that it was to be scrapped, the program will continue into the foreseeable future, spokesman Charles Ross says.

"It will be reviewed in 2006 or 2007 but from the start that was always going to be the case," Ross said in response to news reports that the program would die an early death.

"We will look at the future of the program because new technology is arriving, as well as new emissions systems and low-emission vehicles and see whether Drive Clean needs to be re-defined or re-shaped for the future.''

The program, which was launched in the Greater Toronto and Hamilton areas, and in 2001 and 2002 became mandatory in southern and eastern Ontario, has seen 12 million tests done on most of the 6 million cars, vans, SUVs and pickups on the roads in the test areas.

Up-to-date data are not available for the entire test area, but Ross says smog-causing emissions from light vehicles were reduced by 15.2 per cent in the Toronto and Hamilton area between 1999 and 2001 and carbon monoxide emissions fell by nearly 20 per cent.

"That's equivalent to taking 350,000 cars off the road," says Ross, who notes that the program also covers heavy-duty, diesel-powered trucks and buses in Ontario and all non-diesel-powered vehicles, propane, natural gas or gasoline vehicles, registered in the program area for light-duty vehicles.

Despite its apparent success, the program has come under periodic fire since the Conservative government under then-premier Mike Harris established it.

One study in 2000 reported the same car passed the official test 11 times but also failed it nine times upon being taken to 20 Drive Clean facilities.

Some critics have also suggested the pass rate of 82.9 per cent for light-duty vehicles indicates the emissions standard was too low.

Vehicle owners whose cars must be tested in 2004 are to receive a letter in the mail 75 to 90 days before the expiry of their vehicle licence plate.

They'll be required to take their vehicles to any of the province's 1,700 accredited Drive Clean facilities, which may provide tests only, or tests and repairs.

During the 15- to 25-minute analysis, a certified inspector drives each vehicle on to a treadmill-like device known as a dynamometer that allows the vehicle to be driven on one spot.

A probe is placed in the tailpipe and connected to a gas analyzer and computer that measure the concentration of nitrogen oxides, volatile organic compounds, including hydrocarbons, and carbon monoxide.

After the vehicle is warmed up, emissions readings are taken while it is idling and being driven on the dynamometer.

Test results are compared by a computer to emissions standards for the vehicle and its model year.

Vehicles with safety problems, a missing catalytic converter or that release visible tailpipe smoke will not be tested until they are repaired.

Vehicle owners are given an immediate computer-generated report on whether their car passes or fails.

It analyzes the pollutants being spewed from the tailpipe. On average, about 85 per cent of passenger cars and light trucks pass inspection at their first test, Ross says.

If a vehicle fails, it must be repaired before the owner can renew his licence plate sticker, although you can get a conditional pass if at least some repairs are made to reduce emissions.

Reasons for a failing grade are most often caused by lack of maintenance, which leads to the breakdown of an engine component, says Alvin Timmons, manager of the Ottawa Emission Centre, which runs 40 to 50 Drive Clean tests a day.

"There is no specific part that stands out that regularly breaks down, whether a car is three years old or 19 years old," he says.

"If you don't have regular service, that can lead to a dirty air filter, cracked hoses or a burned-out catalytic converter.

"My advice is to follow somewhat close to your vehicle's maintenance schedule and bring your car to a garage twice a year for a checkup.''

On average, owners whose vehicles fail the test spend $400 to $700 to get a passing grade, he says, although he has seen some repair bills in the $7,000 range.

For more information about Ontario Drive Clean, call 1-888-758-2999 or visit http://www.driveclean.com.