Have this 4.4 V8 with an occasional , slight misfire.No codes logged . Emissions fine at 1500 RPM plus.
The car has seperate cats and exhaust system for each bank of cylinders,(no link pipes) leading to a seperate back box with twin exit pipes on each side (4 pipes total).
At idle,with the sample probe up one exit pipe on one side , the O2 level is higher than i expected ,so i experimented with blocking the other exit pipe (on the SAME side) with a rag.I thought that might bring the O2 level down, but instead that GREATLY increases the oxygen reading.Am i missing something? can someone explain? This occurs on whichever bank/exhaust side you test.
Gas readings below:

N/S (idle, probe in 1 exit, other open)

CO 0.02 %vol
CO2 15.3 %vol
HC 0000
O2 1.15 %vol
Lambda 1.052


N/S (idle, probe in 1 exit, other blocked)

CO 0.02 %vol
CO2 10.1 %vol
HC 0023
O2 07.4 %vol
Lambda 1.516

O/S (idle, probe in 1 exit, other open)

CO 0.00 %vol
CO2 15.3 %vol
HC 0000
O2 1.05 %vol
Lambda 1.048

O/S (idle, probe in 1 exit, other blocked)

CO 0.00 %vol
CO2 09.3 %vol
HC 0000
O2 08.5 %vol
Lambda 1.644


Either side @ 2000RPM exits open

CO 0.00 %vol
CO2 16.1 %vol
HC 0000
O2 0.10 %vol
Lambda 1.004

Thanks a lot

Colin.