Locating Sensor 1 Bank 1

I’m having issues locating Sensor 1. I get the error P2458 but the car won’t regen. I got the readings last night at 2500RPM:

My pattern:
•    Sensor 1: 247 °C
•    Sensor 2: 541 °C
•    Sensor 3: 615 °C
•    Sensor 4: 508 °C

This surly cannot be real gas behaviour; gas cannot more than double in temperature between sensor 1 and sensor 2 without an enormous heat source in between, and certainly not while still under the same exhaust flow.  I assume therefore that sensor 1 is telling the ECU not to begin a regen. 
 

I’d welcome help locating this sensor.