I sure could use some help.
My 04 rx8 has 37k on it.
Shows codes p0037 and p0138, so I replaced both sensors (actually the front one I replaced twice just to be sure) but I still get the cel with these 2 codes!
What can I do?

 

Those codes only refer to the REAR O2 sensor. Follow the diagnosis below and you should find your issue.

P0037 - Rear HO2S heater control circuit low
P0138 - Rear HO2S circuit high voltage

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1. Remove rear O2 sensor connector inspect for poor connection (such as damaged/pulled-out pins, corrosion) since the sensor has been replaced check harness side.
2. Measure the voltage between rear HO2S terminal A (wiring harness-side, top right wire from rear of connector) and body GND with Key On, Is the voltage B+? Yes: Repair or replace the wiring harness for a possible short to power, clear codes and retest.
3. Measure the voltage between rear HO2S terminal D (wiring harness-side, bottom left wire from rear of connector) and body GND with Key On. Is the voltage B+? Yes: Repair or replace the wiring harness for a possible short to power, clear codes and retest.
4. Inspect PCM connection (such as damaged/pulled-out pins, corrosion), if good clear codes, if codes return replace PCM.

Hope that helps

Braden