Need Advice w/ Error Codes on 99 SE

My max has a hard time cold starting and idles at 1,000 rpm even when warmed up. I read the following two error codes:

1. 0802 (P0335): POS Crankshaft Position Sensor or Curcuit fault
2. 0510 (P0138): Downstream O2 sensor maximum voltage monitor fault

What do these mean? Should I just install a new O2 sensor and hope it goes away? I don't want to pay a mechanic for hours of trouble shooting if I can do some of it myself.

Thanks for your suggestions!

Learn from my mistakes. Just take it to the dealer pay little money and they will tell you whats causing the problem. Instead of putting parts on and at the end still not the problem.