Date registered: Jan 2010
Vehicle: Mercedes CLK 230 K
Location: Hemel Hempstead
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the MAF sensor (Mass Air Flow) is a sensor that detects how much air is being drawn into your engine, it is located on the inlet side off the engine between the air filter housing and the inlet manifold. there will be a wiring plug connected to it.
The MAP sensor (manifold absolute pressure) is measuring vacuum pressure again on the inlet side of the engine. (i'm not 100% sure where it is exactly located on your model, sorry, but generally they are connected to a small vaccum pipe which in turn is connected to the inlet manifold.
The Lambda sensor (also called an oxygen sensor, or an O2 sensor) is screwed into your exhaust pipe, is measures the fuel/air ratio and adjusts the fuel mixture accordingly (IE: if it senses that the mixture is too weak it will tell the ECU to add more fuel)
All 3 items have a direct affect on fuel consumption and wont suffer from a clean out once in a while, however if one of the sensors is simply not reading correctly then replacement is the best option, although determining which one if any is faulty is not simple. a diagnostic check with a decent tool should show the readings for each sensor (the dealer or good local garage would have to do this)
If one of the sensors has failed completely you should get a management warning light on your dash, but the sensors that are still working, just not giving the right readings are more troublesome since you dont normally get a warning light, just what you are experiencing.