Q: 2002 Nissan Sentra GXE is smoking out the tail pipe. I replace a knock sensor and the code for the knock sensor still pops up.

HI there. There are different symptoms that create smoke that emits from the tail pipe. Here is a description of the different smoke for you to check:

Smoke Types

• Blue:

Blue smoke is an indication that the engine is burning too much engine oil due to a stuck PCV valve, broken or stuck piston oil control ring, or the valve guide seal on the intake valve is broken or damaged causing engine oil to be consumed.

• Black:

Black smoke is an indication that the engine is burning too much fuel due to a leaking fuel injector, a faulty EGR valve, or a spark plug that is not crating a spark.

• White:

White smoke is an indication that the engine is burning coolant in the exhaust due to either water that has been entered the intake system or the head gasket has a leak making coolant leak into the combustion chamber allowing it to be burned with the fuel.

Check to see what the smoke color is and this will determine what the cause of the engine’s misfire condition is. If the knock sensor was replaced and the light is still coming on, then the engine has a misfire and is causing the engine to shake enough to make the sensor send a signal. If the code was for a faulty sensor, then the wiring could be damaged or frayed making a connection error. If you need further assistance with your tail pipe emitting smoke, then seek out a professional, such as one from Your Mechanic, to help you.