P0031 and P1148 codes
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You are correct. The P0031 code is for the heater circuit inside the upstream air/fuel ratio sensor (similar to an oxygen sensor, but a little more sophisticated). The P1148 code is for the closed loop control, which would be directly effected by the bad sensor. 99% of the time, it's the sensor that is the fault. My advice would be to go ahead and replace the upstream sensor, erase the code and clear the self-learning in the ECM. For the sensor, you will need to find out if the vehicle has Federal or California emissions certification, which will be listed on the sticker under the hood. Rockauto.com sells the sensor. I would recommend sticking with either NTK or Denso brand; Denso will be a little less expensive.
For Federal emissions, you can go with Denso 2349038 or NTK 25679
For California emissions, you can go with Denso 2349036 or NTK 25685
For Federal emissions, you can go with Denso 2349038 or NTK 25679
For California emissions, you can go with Denso 2349036 or NTK 25685
Try clearing the code and then do the procedure to erase the self-learning memory and see if that fixes it, assuming the same code triggered. If it doesn't, then refer to the diagnostic procedure for the trouble code triggered in the Engine Control section of the factory service manual. Nico Club's website has free online manuals at their site you can use.
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Here's the procedure from the FSM to clear the MIXTURE RATIO SELF-LEARNING VALUE:
Start engine and warm it up to normal operating temperature.
Turn ignition switch OFF.
Disconnect mass air flow sensor harness connector.
Restart engine and let it idle for at least 5 seconds.
Stop engine and reconnect mass air flow sensor harness connector.
Using the scan tool, make sure DTC P0102 is detected.
Use the scan tool to erase the DTC P0102.
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