Check Engine Light Error Codes??

[xxvengxx]

I've got a 1999 Infiniti I30 and recently the Check engine light came on.... I have access to a tester and the tester came up with "P0135, O2 Sensor Heater Cicuit Malfunction (Bank 1 Sensor 1). Which O2 Sensor is that?? is it front or rear?? Someone tols me it was one of those but wasn't sure which, he seemed to know it wasn't the one right before the cat though. If anyone has a link or knows which one it is I would appreciate it. Also want to replace the plug and don't know whether to get the double platium or regular platium, I'm getting NGK, just don't know which one to get. thanks for the help

[maxhopper]

Bank 1 Sensor 1 is one near the rear of the engine bay.

The OEM replacement plug is the Double Platinum, part # PFR5G-11. Be sure to check http://www.sparkplugs.com for good prices on the plugs.