Some Ottawa mechanics estimate half of the cars they see are failing the province's new Drive Clean test, not because they are pumping more pollutants into the atmosphere, but due to how data is saved in memory banks of vehicles.

Ontario's new emission test, conducted using a vehicle's on-board diagnostic computer chip rather than measuring exhaust emissions straight from the tailpipe, is creating a lot of work for mechanics and a lot of headaches for car owners.

Lok Ng at Unico Auto Services Ltd. on Gladstone Avenue said many vehicles fail even though they are new, rarely used or recently repaired.

"Here's a 2008 Chevrolet Impala. He come here almost 10 times for testing it," said Ng. "Even the dealer tell him they don't know what's going on."

5 steps to make your vehicle ready for a Drive Clean test

1. Make sure the vehicle has been parked for eight hours without a start.

2. Start the engine and allow it to idle in Drive for two minutes and 30 seconds with the air conditioning and rear defroster on.

3. Turn the air conditioner and rear defroster off. Drive the vehicle for 10 minutes at highway speeds.

4. Drive the vehicle for 20 minutes in stop-and-go traffic.

5. Go to your test.

Ng pointed to another vehicle, a 2009 Honda Fit, which is known for great gas mileage and presumably acceptable emission standards.

It failed an emission test, too, because according to Ng it was boosted, resetting the on-board diagnostic and leading to a failing grade.

Mechanics drive-testing failing vehicles

Down the street at Lorelli Service Centre, mechanic Jim Sanidas simulates daily activities in a customer's vehicle, such as dropping kids off at school and picking up groceries, to build a background of emissions data so he can scan the vehicle.

"We've been driving that Mercedes three or four days now and it still hasn't come ready," said Sanidas.

Volkswagen owners are privy to online instructions to prepare for the on-board diagnostic emission test, which includes 17 specific steps, similar to the five steps provided by the Ministry of Transportation for the Drive Clean test.

Volkswagen's steps include accelerating and decelerating to speeds between 0 km/h and 100 km/h, holding speeds, stopping, starting and repeating the process.

Sanidas's is to book the emissions test two weeks before you need it because there can be a long process to get the Drive Clean stamp of approval.

"Basically you have to give the car back to the customer, tell them to drive it for a week under different conditions, get the monitors ready ... then you can go through with the test," he said.