Do I need a reset
 

After replacing the petrol and oxygen Lambda sensors do I need to do a reset to extinguish the engine management warning light ? Or will the system management do it automatically ?

Ray, the short answer to your question is yes and no. The light will not go out immediately. You will need to start the car and drive for about 10 miles five times. On the 6th start (assuming the previous problems have not shown themselves in the previous 5 drives), the light will go out and you will get the P1111 code.

Now, if you reset the codes and go to a P1000 status (ie, no checks have been performed), the light will be out until the P1111 status is achieved (1st drive after about 10 miles in some cases, may take longer depending on the variation in speed and engine RPM). If any code is seen during this time (ie, before the P1111 status is met), the light will turn on and you will get that code and you will then have to reach the 6th start without the problem re-occurring.

I am not trying to make this confusing, but the computer uses a few different trains of logic to determine if it needs to turn the light on or not.

Thanks thermo, as usual ever reliable and willing to give the advice required.