I always have customers coming into the shop asking questions about what the check engine light really means when it illuminates, what causes it and how does the car computer really operate. Below helps to explain just that. Recently I had a customer come in with a vehicle issue. His vehicle was running rough, stalling and hard to start after warmed up. I determind the problem to be the Tempurature sensor. When it failed, the computer was reading the engine Tempurature when warmed up to be 62 degrees, when in reality, it was 200 causing the computer or PCM to take over the operating system since it thought the engine was still cold. Similar to if your choke on Carburated engines was staying on. So I explained to him as you will read below, just how the vehicles computer tries to adjust the problem.
The car computer or powertrain control module (PCM) is a designated computer that was developed to manage the engine control system. The PCM consists of electronics which are mounted on a multi-layer circuit board. The PCM monitors and adjusts the air/fuel mixture and utilizes a catalytic converter to minimize the amount of pollution produced from the engine. There are two modes of operation, closed loop, which means the computer has completely taken over the operation system. And open loop which is used when the engine is cold and operates on a preset program. The engine must be at operating temperature before it can go into closed loop.