I've just recently gotten my 818 driveable and legal and am supposed to get it tuned next week. The check engine light is on and my Accessport says it's a P0031 code. My research shows there's some issue with the front A/F sensor heater circuit. I can't seem to find the issue despite a week of troubleshooting.

818 Particulars:
-Dieted donor harness
-Factory exhaust
-Running both the front A/F sensor and rear O2 sensor; A/F sensor is NEW (Denso)
-Tune is a base tune from the tuner for my combination (2.5L hybrid, dark blue injectors, VF39, Walbro fuel pump)

What I've Done:
-I have the factory manual and followed the P0031 troubleshooting
--At sensor, voltage of yellow-red wire reads 12-14V
--At ECM, grounds checked good on B84, Pins 8 & 9
--At ECM, output signal on B84, Pins 4 & 5 showed a few tenths of a volt (very hard to check with the connector attached to the ECM; I stuck small wires in the back of the connector as pigtails to my multimeter)
--Resistance across heater circuit on A/F sensor is 1.5 ohms

Everything seems to check out following the troubleshooting. I checked voltage of the white wire at the sensor, which should be the signal wire, and got 3.5V. Sensor was unplugged. This is different than the <1V I got at the ECM on pins 4 & 5. I was expecting to see the same voltage at both places.

All of the wiring is completely covered by firewall, center console, etc so before I start taking things apart, I was hoping someone on the forum had experience and could lend some advice. Thank you for any insight anyone can give!