What does it mean?
For the internal combustion engine to function, the valve system opens and closes the cylinder at different points in the cycle. On the intake stroke, for example, the intake valves should be open, allowing air and fuel to enter the cylinder. On the compression and power strokes, both intake and exhaust valves are closed. Finally, on the exhaust stroke, the exhaust valves open to allow the exhaust to escape through the exhaust manifold.
Granted, this is an oversimplification, because we have to recall that the cylinders are moving fast, so the valves actually open and close before or after those strokes have begun and ended. In the typical engine intake stroke, the intake valves open (IO) 8° BTDC (before top dead center), allowing air and fuel to enter from the intake manifold, and the exhaust valves close (EC) 10° ATDC (after top dead center).
On the compression stroke, the intake valves close (IC) 44° ABDC (after bottom dead center). The spark plug fires, and the power stroke forces the piston down, and the exhaust valves open (EO) 50° BBDC (before bottom dead center). The exhaust stroke forces spent gases out the exhaust manifold, and the cycle starts again. This works fine on a typical medium-performance engine, such as found in any sedan on the road today.
On the other hand, performance engines, perhaps in a sports car, would benefit from different “breathing” habits, and performance engineers discovered that, by changing valve opening and closing angles, they could squeeze more power out of the engine. A performance engine’s valve opening and closing angles might be something like this: Intake IO 17° ATDC & EC 17° ATDC, Compression IC 59° ABDC, Exhaust EO 59° BBDC.
Engines destined for performance applications might get a cam profile like this, but the engine would not perform as well when cruising, for example. To get the best of both worlds, performance on demand and cruising fuel economy, most modern engines are equipped with variable valve timing (VVT). Depending on engine speed and load, the engine control module (ECM) adjusts valve timing, some VVT systems affecting only intake valve timing, while others varying both intake and exhaust valve timing.
To control valve timing, the ECM modulates a VVT solenoid valve. In turn, the VVT solenoid valve controls engine oil pressure application in the VVT head, mounted on the camshaft. Depending on system design, VVT can advance or retard valve timing.
If the ECM detects a fault in the VVT solenoid valve circuit, such as voltage out of specification, it will set a diagnostic trouble code (DTC), P0079 Exhaust Variable Valve Timing (VVT) Control Solenoid Circuit Low (Bank 1), and illuminate the check engine light (CEL). Keep in mind that Bank 1 refers to either an inline engine (most four- and five-cylinder engines, and some six-cylinder engines) or the bank of cylinders containing cylinder #1 (V6, V8, and V10 engines).