Before buying the lamda sensors I would try and look for air leak or vacuum leak to rule out cheaper option
I would take the car aout for a good run to burn carbon of the cats and sensors
double check vacuum leaks then look at Lambda sensors as I doubt if both sensors would go down at same time
Check the two caps No. 15 and 17 in above diagram at back of inlet manifold, you can do this by running your hand at back of manifold near bulkhead and feel if caps are perished or not.