What is Bank 1?

[matt79]

I scanned the check engine light. I got code PO 420. Which says catalyst efficiency below threshhold (bank one). So where is bank one located? I'm guessing it is the O2 sensor on the rear exaust manifold...

[GTP Dad]

Welcome to AF!!

Most of the single exhaust cars only have bank one. Some do have two banks but in your case I suspect you either have a cat issue or the O2 sensor is bad. The sensor is easier to change but if the code does not go away you will need to change the cat.

[MasterTech8]

On a V6 or V8 engine bank one is the side of the engine that cylinder one is on. Then bank two is the side of the engine that cylinder two is on

[xeroinfinity]

Actually if it is a V8, bank 2 is on the right (passenger side).
Sensor 1 is always before the catalytic converter (sensor 2 is after)

[Ron G.]

In the 3.1 engine (transverse mount) bank one is at the aft (firewall) side.

In the 3.0 & 3.3 engines (transverse mount) bank one is at the fwd (radiator) side.