If your vehicle is showing blue or gray smoke coming from the exhaust, here are the most likely causes and the parts you'll need to fix each one.
A stuck or clogged Positive Crankcase Ventilation (PCV) valve can cause excessive crankcase pressure, forcing engine oil vapor directly into the intake manifold where it is burned.
If too much oil was added during a recent oil change, the excess oil can foam up and be forced past the piston rings or into the PCV system, causing it to burn in the cylinders.
Valve seals prevent oil in the cylinder head from leaking into the combustion chamber. When they harden and wear out, oil drips down the valves while the car sits, typically causing a large puff of blue smoke upon startup.
Piston rings seal the combustion chamber. Worn rings allow oil from the crankcase to bypass into the cylinders and burn, usually causing continuous blue smoke while accelerating or under heavy load.
On turbocharged vehicles, worn internal seals can leak oil directly into the intake tract or exhaust system, where it burns off as blue or gray smoke.
A severe head gasket failure can allow a pressurized oil passage to breach directly into a cylinder, burning large amounts of oil. This is usually accompanied by poor engine performance and misfires.