P0037 Info - Post-cat O2 Sensor Heater Circuit
 

Just a quick thread to save someone some time.

P0037 - open or shorted sensor heater circuit.

Symptoms: Check engine light, no other engine codes, significant drop in highway mileage (ex. my mileage dropped from 28 to 21 when this light turned on), and it felt like I lost some power. Happened to me at only 30k miles, but this is a heavily tracked car.

Diagnosing: In addition to the P0037 code and drop in mileage, you can confirm, with the car off and sensor disconnected, by measuring the resistance of the heater circuit. The post cat O2 sensor has a black connector on the front right of the engine, near the fuse box and coolant reservoir. There will be a matching grey connector for the pre cat O2 sensor right next to it. Check the heater circuit by measuring resistance between the two pins connected to black wires; a brand new sensor will measure 6.5 ohms; my failed unit was open circuit.

Part:
OEM part is made by Denso. Same part for 2013-2020, manual and auto. This sensor has the cylindrical tip, black connector, and come with two fir-tree cable holders to fasten the cable to the head.
Subaru: 22690AB000 $199.95 MSRP
Toyota: SU00300428 $170.80 MSRP
Denso: 234-4908 $49.79 (Rockauto.com)
NTK: 24642 $55.79 (Rockauto.com)

Fix: Replace the post cat O2 sensor. Just push the connector tab down and pull the connector, pull out the two fir tree cable ties out of the head(plastic push rivet removal tool helpful here), and unscrew the O2 sensor. I had to lift the car and remove the aluminum panel, but I don't remember accessibility for OEM because I have headers. Installation is the same but in reverse. Clear the code, and you're good. Sensor hex is 22mm. New sensor should come with pre-applied anti-seize; apply some if it is not present. O2 sensor torque value is 22.1 ft-lbs or 30Nm.

.. or, delete it and disable it in the tune and in its relative DTCs..