I think you mean code P0137, which is Bank 1 Sensor 2. This is the after cat heated O2 sensor on the firewall head. This is an easy replacement, although a stealership will likely charge you $50-$100 labor, plus huge $$$ for a Denso sensor when the aftermarket Bosch is cheaper and most likely superior in design.
If you pull your van up on ramps, you can slide under and find this sensor. It's located right above the exhaust 'Y' pipe where the 2 heads are exhausted together. If your van is AWD, the rear driveshaft will be partially blocking your view; this is not a problem, you can still reach it. Now, you will notice that just after leaving the sensor, the wire is clipped to the undercarriage (to keep it from snagging on the road, rear driveshaft, etc.) and then it goes through the sheet metal near the left front tire via a grommet. Gently pry the plastic wire clip out (use a small screwdriver) and try to save it for use on the new sensor; it's designed much like a toggle bolt and held to the wire with a short zip-tie, which is easily removed once the old sensor is out. The wires then go through the grommet and reappear inside the passenger cabin. Open the driver's door and put the driver's seat all the way back to give yourself some room. Now, remove the small plastic kick panel immediately to the right of the accelerator pedal. It has one push clip near the firewall and 2 hidden edge clips where it connects to the main console kick. Remove the push clip and disconnect the 2 hidden edge clips by gently pulling the small panel toward the firewall. Once the kick panel is out of the way (you may need to pull the edge of the carpet back a little) you'll see the other side of the grommet. The connector pigtail is just above the grommet (you may need to twist your head up in there to see it, but follow with your fingers - it's there, mounted to the metal console frame). Gently disconnect the pigtail but pushing the lock. Now push the grommet, wire and pigtail through to the underside. Turn out your old O2 sensor (7/8" open end wrench works well, if not seized from heat) and your old sensor is out. Installation is the reverse procedure. Don't forget to put anti-seize on the new O2 sensor threads (if not already on from the factory), and to untwist the wires after you turn it in.