There are 2 main operation modes for the ECU.
Open LoopOPEN LOOP - this is the mode your ECU is in until the coolant temperature and a couple other sensors get to operating values. The ECU also runs the car in open loop when you are at Wide Open Throttle (WOT) or if there is an error condition causing a check engine light.
Limp ModeOpen Loop can place the car into a fault condition called Limp Mode. The concept is that something is wrong and the ECU will allow you to 'limp home'. This means, rpm limit around 3k and pulls timing/fuel to generate just enough power to drive it home. This condition helps protect the engine components from further damage and saves you a tow to bring it home or to a shop for repair. If you are in limp mode, your Check Engine Light (CEL) or Service Engine Soon (SES) indicator is likely on. You can figure out what sensor is causing the error by reading the code. This article will help pull and translate the code.
Closed LoopCLOSED LOOP - the ECU uses information primarily from the Oxygen Sensor and adjusts fuel delivery and timing to achieve minimum emissions, better gas mileage and power.
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