The powertrain control module, aka PCM, is your vehicle’s brain. It manages the engine, transmission and other systems based on information it receives from various sensors around the vehicle. It is just one of dozens of microprocessors on today’s vehicles that run everything from the climate control system to the power windows.
The PCM receives signals from sensors that measure air flow into the engine and out the exhaust, the coolant temperature, how far down the accelerator pedals is, the speed at which each wheel is turning and other parameters.
It then uses that information to make dozens of decisions per second, such as how much fuel to inject into each cylinder, when to fire the spark plugs and when an automatic transmission should shift to a different gear to deliver the best performance for the current conditions.
Some vehicles refer to the PCM as the “engine control module” or “electronic control unit.” Others, mainly older ones, have separate control modules for the engine and automatic transmission that stay in constant contact with each other — like BFFs on their iPhones — because the transmission reacts to what the engine is doing and vice-versa.
When this flow of information between the PCM and other onboard computers and sensors works properly, the result is smooth and efficient performance, with the PCM seamlessly making necessary changes without any fuss.
But as fast and smart as PCMs can be when all is well, they can be dumb as bricks when things go wrong. For example, if an oxygen sensor in the exhaust system conks out, the PCM will be scratching its digital head and unable to figure out how to adjust the air-fuel mixture going into the engine. The check-engine light will probably go on — a cry for help — and the engine might run roughly, have sluggish acceleration or other issues. Likewise, if the PCM fails, the engine won’t start.
Because PCMs rely so heavily on inputs from other sources, they sometimes are falsely accused of causing a check-engine light or engine problem. Even if the trouble code for a check-engine light points to the PCM, mechanics test PCMs and often check inputs to make sure it is the culprit before replacing it.