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Your vehicle failed a smoky exhaust test

Published: | Category: Vehicle choices , Leaflets & brochures | Audience: Motorists

This page is for vehicle owners whose vehicles have failed awarrant of fitness (WoF) or certificate of fitness (CoF) inspection because of a smoky exhaust. It contains information on the causes of exhaust smoke that may help the owner have the vehicle repaired so that it can pass its WoF or CoF.

Publication details

  • Author:
  • Published: 25 October 2006

Why has my vehicle been tested for smoke emissions?

The government has introduced new requirements as part of an ongoing campaign to reduce vehicle emissions and improve public health and the environment. From 27 October 2006, most vehicles cannot obtain a warrant of fitness or certificate of fitness inspection if they produce clearly visible smoke from the exhaust tail pipe.

How is my vehicle inspected?

During the test, the visibility of smoke is assessed when your vehicle’s engine is idled for five seconds, and then accelerated to the lower of 2,500 rpm or half the maximum engine speed.

What if it produced visible smoke when it was new?

Vehicles with two-stroke or rotary engines, oldervehicles (pre-1960) and later vehicles that produce some visible smoke becauseof their design may be allowed to produce minimal smoke. For these vehicles, the smoke produced must not be noticeably and significantly more visible than it would have been when the vehicle was manufactured, and running on the fuel recommended by the manufacturer. An engine maintained in original condition should pass the test.

For a later vehicle (post-1960) that produces some visible smoke because of its design, you need to have documented evidence to prove this. Acceptable evidence includes a letter from the manufacturer or their representative, an automobile club or an appropriate expert such as a diesel engine specialist.

Can my vehicle be fixed?

Below is a guide to what may be causing your vehicle’s engine to smoke.

Some causes can be easily fixed, eg, by changing the air filter. Sometimes the vehicle may require a simple tune-up. In other cases, the vehicle’s engine condition may be more serious and require extensive repairs.

You should consult a mechanic with experience in this area to ensure the most effective repair for your vehicle.

Type of smoke Diagnosis Possible causes
Petrol-powered vehicle
Black or grey Incomplete fuel combustion
  • Air filter clogged
  • Carburettor, choke, fuel injection or emission system malfunction
  • Ignition timing wrong Manifold blocked
Blue Engine oil being burned
  • Engine wear (pistons, rings, cylinders, valves, guides, seals)
  • Turbo pressure seal failure
  • Engine oil level too high
Diesel-powered vehicle
Black or grey Incomplete fuel combustion
  • Air filter clogged
  • Timing incorrect
  • Engine overheating
  • Injection system faulty
Blue Engine oil being burned or atomised
  • Engine wear (pistons, rings, cylinders; valves, guides, seals)
  • Engine oil level too high
White smoke Fuel not burning
  • Injection system faulty
  • Low compression
  • Timing incorrect

What if I disagree with the decision?

If you disagree with the decision to fail your vehicle, please try to first resolve the issue with the inspecting organisation concerned. If you are still not satisfied, you may raise a complaint with the NZ Transport Agency, phone 0800 699 000 or download a complaint form.

Where can I get more information?

You can find more information on the effects of exhaust emissions and tips on reducing them on the Ministry of Transport’s website: link)