HI there
For these codes you are looking for the upstream O2 sensor. Both codes are related, P0031 is the code that determines which sensor. Since it's a 4 cylinder, you don't have bank 1 and bank 2, you only have upstream and down stream. You need the upstream sensor.
P0031 NISSAN - HO2S11 Heater Control Circuit Low Read more: https://www.autocodes.com/p0031_nissan.html
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