My 2014 ES 350 has the CEL on and the DTC being displayed is P0051 which is Bank 2 sensor 1 air/fuel sensor Heater control circuit low. When the code is cleared it comes back within 10 seconds of starting the engine again.

I have tested the green white and red wires of the sensor plug back to the ECM plug and they test good continuity to the proper pins. I have tested the larger gauge green wire back to the fuse block and it shows good continuity. With the car on but engine not running I test 12v on the larger green wire. I test 3.3v on the one sensor circuit wire and 3.0v on the other sensor circuit wire which is correct according to a manual that I found and some you tube videos. This was measured at the harness plug with the sensor unplugged.

This leaves the white wire that goes back to the ECM and should be a pulse modulated ground that cycles to create the needed current to maintain 1200 degrees in the heater circuit. I don't have an oscilloscope to test this signal correctly, but my meter does show voltage readings coming and going on the heater wires as if the ground is Being cycled. This test was done with the sensor connected and back probes inserted into the sensor plug while the engine was running.

Even though the sensor tested a good 2.9 ohm resistance on the heater circuit, it was not a Denso sensor so I got a new correct Denso a/f sensor today and installed it. The code returned within seconds.

I believe with the sensor being replaced and with the wiring testing good, the next thing to do is to get a new ECM. Does anyone have a suggestion for a test that I have not tried, or some experience with this code and what it took to clear it?

 

I replaced the ECM with a used one and got it reprogrammed and the CEL light remained off while it idled for quite a while. I was thinking the problem was gone, but then when I started to drive it, the light came right back on!
So the sensor is a new Denso A/F sensor, the wiring to the sensor tests good and the ECM has been replaced. I am wondering if the A\F htr relay might be the cause, but I can't seem to find that relay. The schematic shows the 12v power to bank 1 and bank 2 upstream a/f sensors comes through a relay called the a/f htr relay located In the front left section of the engine compartment, but I do not see a relay labeled as that under the hood anywhere.

Does anyone know where this relay is located?