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Download the factory service manual for your model year Frontier.

If you replaced all your O2 sensors, did you use stock Nissan OEM O2 sensors or did you use aftermarket. Were those aftermarket O2 Sensors Bosch or Denso?? PO031 code refers to a faulty Heater Control Circuit on you O2 sensor.

You either have a short in the wiring harness leading to the Oxygen Sensor or your have a bad Oxygen Sensor....

Hope this helps....Good Luck....
 

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Do you have an OBD II Code Reader that you can connect to your Frontier to see if you can monitor the voltage / signal coming out of your Oxygen Sensor?? If you do, what kind of readings are you getting?? Are those readings matching up with what's listed in your model year Factory Service Manual for your Oxygen Sensor?? If you are NOT getting any readings coming out of your Oxygen sensor to the Engine Control Unit, or if the reading are not within specifications, your ECU is going to trigger the PO031 Code. You are going to have to trace the wires inside your wiring harness & the ground wire from the ECU to the actual Oxygen sensor You'll need a volt meter & continuity tester to check the wiring harness, connector & ground wire for voltage & shorts in the wiring. If any of those wires are bad, you kick out the PO031 code. You'll also need one of those non-contact heat thermometer, (you can pick one up a Home Depot or Amazon), to see if the heater circuit on your Oxygen sensor is getting voltage & heating up like it's supposed to be doing during startup / warmup.
 
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