Which StepBox Cover Do I Need for My 589?

If your Peterbilt 589 has the battery box on the driver side and it’s a weed burner (exhaust dumps under the truck)
You can use the same box cover on either side.

But if your truck is a DPF-style with the exhaust can (DPF) on the side,
You need the DPF-specific box cover for that side the battery box one wont fit.

Quick Rule
Weed burner = same cover both sides
DPF on side = use DPF box cover on passenger side