2001 Sienna OBD II Code Questions

The "check engine" light on my 2001 Sienna recently flashed on, and, before I had a chance to intervene, my wife took it in for its annual safety inspection and emissions test -- which it of course failed. Checked the codes and got: P1130 - A/F Sensor Circuit Range/Performance and (like the previous poster) P1155 - A/F Sensor Heater Circuit. Been poking around on the internet, and I've learned that the 1130 code might not necessarily indicate a bad sensor, and I also get the impression (though it's hard to decipher some of what I've found) that the 1155 code might be directly related to the 1130 code (that is, one may be causing the other -- or both by a third problem). Any insight into this issue would be greatly appreciated since I'm not too excited about spending $200 to $300 replacing these parts if the part is not the problem...

Thanks!

Kevin

P1130 A/F sensor circuit range/performance malfunction (bank #1 sensor #1). It's the sensor on the manifold closes to the firewall. Had that same fault last month.