replacing O2 sensor

I need some help from the Ford experts.
I have a 1999 E-150 with the 4.2L. One of the O2 sensors are bad on the left bank, but I am not sure which one. My question is: Does anyone know where exactly the O2 sensors are on these Vans.. I figure the rear will be easy to find, but what is the easiest way to get to the one before the cat? Will I need any special tools to get to it?

Thanks for any help.

how did you come to the conclusion that the sensor was bad?
as far as location there are two left bank sensors one before the cat and one after find the cat converter and just follow the exhaust pipe.
to determine which sensor is showing a fault have the codes pulled they will indicate which sensor is at fault.

how did you come to the conclusion that the sensor was bad?
as far as location there are two left bank sensors one before the cat and one after find the cat converter and just follow the exhaust pipe.
to determine which sensor is showing a fault have the codes pulled they will indicate which sensor is at fault.


I had the codes pulled already.. all it told me was left bank. I may have them pull it again to make sure. Will I need to remove the plastic cover in the van to get to the O2 sensor before the cat.. I know they are typically on the exhaust manifold.