Many thanks Andrew for your help and indeed I think the penny has dropped now!. Great pics along with explanation too. So I gather the first sensor is the same as the last one so guess that is why I have an odd reading of NaN% for the bank 1 2 sensor. The sensor in the first cat at the bottom is just a heat sensor I guess. Will try to order one guess if I order the same part code as the first one guess will fit the last one. Did you get your inlet flap sorted? rgds andrewHi,
items 20 and 21 as per the above diagrams are the same thing, there are actually two different diagrams in the earlier post with different numbers.
The top diagram shows the placement of the sensor on the exhaust, number 20. The bottom diagram is how the same sensor, number 21 here, connects to the ECU. The NOx sensor is under the car and you can see it easily. It is to one side of the pipe and angled slightly upwards. I have changed this sensor in my first FSI. The wires goes through a hole in the floor in the passenger footwell and to the emissions control unit. This is a separate box to the engine ECU and is bolted to the floor. The wire goes from the exhaust and is clipped above the heat shield and the place where it enters is covered with a plastic cover that protects where the engine wiring also enters the car. This will need to be removed by undoing the retaining nuts and bolts. The gromet is part of the cable and this just pops out which allows the plug from inside the car to go through the hole to the ouside.
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The NOx sensor is actually a combined sensor and acts as both a lambda probe and NOx sensor. It therefore costs more than a regular lambda probe. The lambda probe part forms the usual post CAT lambda check, and mostly is used to verify the correct operation of the calaytic converter. The second lambda sensor acts in the same way as in the FSI as it would do in a "regular" petrol car. If the lambda part is broken then the emissions control engine warning light will be on as the car can't verify that the emissions are correct. In my car, the lambda sensor part was OK but the NOx part wasn't working so the warning light was off but a fault code was logged for NOx. Unfortunatelly for me, after changing my sensor, the car then tried to run in lean burn mode and instead just told me that my inlet flap actuator was broken. :-(