Visible Smoke
A properly operating gasoline-powered vehicle does not emit visible smoke once it's warmed up. Different colors of smoke indicate different engine problems. A smoking vehicle will usually use more gasoline than one that is repaired, costing you money. Smoking vehicles can generate 10 to 15 times more pollution than well-tuned vehicles.

Gasoline Engines*

Smoke Color	Diagnosis	Probable Causes
White	Coolant leaking into combustion chamber	Bad head gasket Cracked block or cyliner head
Blue	Engine oil being burned	Oil leaking into combustion chamber Worn piston rings, valves or cylinders Bad exhaust manifold Bad head gasket
Black/ Grey	Incomplete fuel combustion	Clogged air filter Carburator, choke, fuel injection, or     emission system malfunction Ignition timing off Low compression due to engine wear

Diesel Engines*

Smoke Color	Diagnosis	Probable Causes
White	Improper air/fuel mixture	Faulty fuel injection system Incorrect fuel injection and valve timing Engine overheating Faulty fuel pump and/or injection pump
Blue	Engine oil being burned	Excess engine oil Worn piston rings, valves or cylinders
Black/ Grey	Incomplete fuel combustion	Damaged air filter Faulty fuel injection system Clogged air filter Wrong grade of fuel Incorrect fuel injection pump timing Engine overheating Low compression ration

* Source: Texas Commission on Environmental Quality, "When You Care For Your Car, You Care For The Air", 2002

Tampering
Since 1975, equipment to reduce emissions has been included on most vehicles. Federal and state law does not allow individuals to tamper with this pollution control equipment.

What is "tampering?"
Tampering is when someone removes, disconnects, alters, damages or in any way renders this equipment ineffective. This would also include using replacement parts that aren't equivalent to the original or using parts not originally certified for the vehicle (for example, dual carburetors to replace a single carburetor).

Why do people tamper?
Some people still believe that tampering with a car's emission controls will improve the vehicle's performance. Changing the manufacturer's recommended settings for the engine may actually reduce fuel efficiency. Today's automakers design cars to meet the best possible balance between performance, mileage and low emissions.

Most vehicles in Pennsylvania will be required to undergo some sort of check for tampering:
1996 and newer vehicles in 17 counties are required to have on-board diagnostics tests which by their nature will detect tampering;
1995 and older vehicles in 25 counties with separate emission inspections are required to have visual checks of the catalytic converter, exhaust gas recirculation (EGR) valve, positive crankcase ventilation (PCV) valve, fuel inlet restrictor, air pump and evaporative control system components. Inspectors will check for their presence, whether they are properly connected and whether they are the appropriate type for the vehicle.
a similar visual check will be performed as part of the safety inspection in the 42 other counties without emission inspections.

Classic, collectible and antique vehicles are not required to have visual inspections in any county.

How does a vehicle owner find out what pollution control parts were part of the vehicle's original configuration?
We strongly suggest you work with a qualified mechanic. Sometimes making that determination for older cars can be difficult. Standard reference manuals, such as Chilton's, can be of assistance, but professionals may have access to subscription services that are too expensive for individuals. Vehicle owners can replace missing pollution control equipment with aftermarket parts or used parts as long as they are made to work in that vehicle. You do not need to buy a part made by the manufacturer who made the car. When buying a used vehicle, ask the seller to demonstrate that all the pollution control equipment is present.

OnBoard Diagnostics
The engines in today's vehicles are largely controlled by electronics. An on-board computer controls all of those systems and is capable of monitoring the vehicle to detect a malfunction or deterioration that will affect your vehicle's performance, fuel efficiency or emissions. The computer can detect these problems well before the driver becomes aware of an effect on driveability. This system is called "on-board diagnostics" or OBD.

OBD can detect problems that may not be noticeable upon visual inspection because any component failures that impact emissions can be electrical or even chemical in nature. A light with a "check engine" or "service engine soon" will illuminate, warning the driver of the problem. The car's computer will store a "trouble code" to provide a technician with potential repair information.

Vehicles of model year 1996 have OBD systems that are reliable, standardized and accurate enough to include in state vehicle emission testing programs. Emission testing programs also check whether all components, such as bulbs and the software to illuminate "check engine" lights, are working properly.

EPA's OBD Fact Sheet