Variable valve timing systems have been around since the 1960s in one form or another, but they didn’t start seeing widespread use until the 1990s, when the DOHC system became prominent. They’ve advanced a lot since then, and today, most automakers have their own proprietary variable valve timing system. The variable valve timing switch is a pressure switch, and is designed to be open on a normal basis. It’s located at the VVT solenoid inside the engine and is responsible for telling the car’s computer if oil pressure was delivered to the VVT system. After pressure has been delivered, the switch closes, creating a voltage drop to 0. The car’s computer interprets this as a positive signal of oil pressure delivery. If the variable valve timing switch fails or reads incorrectly, it can cause the engine to run rough and fuel economy will be affected. The Check Engine light will come on with variable valve timing codes.