I take it you mean P0051??
P0051 means - HO2S HEATER CONTROL CIRCUIT BANK 2 SENSOR 1
This translates to the CIRCUIT for the OXYGEN SENSOR (Lambda), ignore the Bank 2 its about which side the cylinders are on (V engines)ie Bank 2 is Cylinders 2+4, Bank 1 1+3, SENSOR 1 means the SENSOR before the CAT.
Its either a faulty lambda sensor or the wiring/plugs, most likely the sensor!

You can see some EOBD 2 Codes here -
www.gunson.co.uk/downloads/77004_en.pdf
The codes list has increased with time so not all are there.

Hope this helps.

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