Had new cats fitted. Transferred the existing 02 sensors checked and OK prior to work and checked with mapping tool showing their activity after the new cats. All A OK. But now have EML light on again - same code P1647. Engineer totally baffled. Emissions are fine, the sensors are seen working but the EML won't go out. It appears to suggest that the topmost 02 sensors are not functioning but activity graphs/oscilloscope show they are working just fine . Anybody any suggestions??

Cheers

Check for a poor electrical connection in the plug.

have you cleared the codes? do they come back? if so, it could be a dying sensor, how many miles on the car?