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Sep 5, 2016

5 Things That Can Make Your Car Fail Its Emissions Test

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If you are a driver in Ontario or British Columbia, you are hopefully aware of the law requiring you to test your car for emissions every two years. The Ontario government introduced the “Drive Clean” law in 1999, to weed out vehicles spewing out unacceptable amounts of exhaust fumes into the atmosphere, causing smog and increasing pollution. So, while this biennial visit to a Drive Clean facility and the few bucks you have to pay for getting a clean bill of health for your car may be a bit of a nuisance, it’s a small price to pay for contributing to a better environment.

Failed Car Emission Test

It’s certainly important for you that your vehicle passes the emission test—a failed test means you can’t renew your vehicle registration, and if you’re trying to sell your car, it’s a dead-on arrival-proposition for any prospective buyer.

The Drive Clean test entails reading the emissions computer OBDII (on-board diagnostics) system. To give you a heads up before your next emissions test, let’s review some of the more common problems that could give your car an “F” on this test.

Before we start, here’s a tip: if your car’s engine light comes on, don’t ignore it, it’s a good sign that something may be wrong with the emissions system. You may be able to still start and drive the car, but there are a hundred underlying problems that could be causing the light to go on—the emissions system being one of them. You need to take the car to your mechanic as soon as possible.

If a vehicle has failed the emissions test, a mechanic will focus on the following areas in the diagnosis process:

  1. Oxygen Sensors: This is a very common problem area, and the mechanic will probably take a look at this first. These sensors measure the amount of oxygen in car exhaust, and if they are not working properly, there is not enough oxygen present, and that will definitely make the car fail its emissions test. These have to be replaced.
  2. The Ignition System: The issue here may be as simple as a defective spark plug wire, and that’s an easy fix. But, there could also be problems with the distributor cap and ignition timing. A mechanic will be able to diagnose what the actual problem is.
  3. Air Injection Systems: These systems reduce harmful emissions such as carbon monoxide and hydrocarbons into the atmosphere by injecting air into the engine exhaust. If high levels of these gases are emitted by a car, the air injection system is usually at fault.
  4. Vacuum Leaks: The first place mechanics look into when they suspect a vacuum leak in places such as gaskets or vacuum lines, is the MAP sensor—the manifold absolute pressure sensor plays a very role in regulating air and fuel. When MAP detects that there is not enough air or fuel, it sends a signal to the engine computer to increase one or the other. With a malfunctioning MAP sensor, the car will not properly control its internal combustion.
  5. Evaporative Emission Control Systems (EMCS): These stop gas fumes from being released into the atmosphere through vehicle emissions. To find out why these systems are not working properly, the mechanic will look at the functioning of the gas cap, purge valves, vacuum hoses and vents.

It’s not all that complicated to avoid having your car fail its emission test—a regular maintenance visit to your mechanic should do the trick. Getting through an emission test with flying colours is one benefit to these visits; the other include a long shelf life for your trusted wheels, and no need to shell out any money for a CAA membership!


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