What does it mean?
The engine control module (ECM) in the modern automobile continuously monitors and modifies engine operation to deliver the most efficient power. Using sensors, such as the crankshaft position sensor (CKP), and actuators, such as the fuel injectors, the ECM can also determine if something is so far out of specification that it can’t compensate. In this case, diagnosis and repair would be necessary to bring the engine back into specification.
Cylinder balance is one method the ECM uses to measure engine efficiency. You see, even if the engine is running at 2,500 rpm, that’s really only an average. With each cylinder firing, the crankshaft accelerates past 2,500 rpm, at that moment, and decelerates below 2,500 rpm. In the typical four-cylinder engine, there’s an impulse every 180°, while in an eight-cylinder engine, the impulse is every 90°, which explains why a V8 typically runs smoother than a V6 or an i4.
Using the CKP, the ECM can detect slight variations in engine rpm, that is, the acceleration of each power stroke, and can determine how much each cylinder is contributing to the engine, that is, the balance between each cylinder. If one or more cylinders doesn’t give as much impulse to the crankshaft, the ECM determines a fault, sets a diagnostic trouble code (DTC) in memory, and illuminates the malfunction indicator lamp (MIL).
In case the ECM is finding fault with cylinder 2, DTC P0266 is defined as “Cylinder #2 Contribution/Balance Fault.”