| They check CO, not CO2 with the probe into the exhuast header. A knock sensor detects detonation in the engine and adjusts the FI to stop it happening as detonation will destroy your engine. A lambda (or oxygen) sensor is fitted in the exhaust just before the cat usually and measures the oxygen content of the exhaust gasses. It then feeds back to the FI system which adjusts the mixture etc to achieve the correct lambda value. | 
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