Hey all,

Having an issue with a 2000 XJR. The car smelled to be running very rich, and on scanning it had P1646 and P1647, the MIL was not on weirdly enough. I tested both O2 heaters and they had infinite resistance.

I had a generic bosch sensor so I tried putting it in the bank A and P1646 disappeared. So I removed the bosch one and bought 2 Denso sensors and installed but I still had P1647. I thought maybe the new Denso sensor on Bank B was a dud so I installed the Bosch sensor in bank B and P1647 cleared but caused the fault to jump to the other side and P1646 came back on.

I've tested all three O2 sensor heaters and all have about 3.5ohm. Tested them on bench supply and they all heat up and pull similar current. I've resistance tested all wires from the ECU to the sensors and they are all fine. Changed the O2 heater relay. If I clear the fault, and start the car, both sensors pull about 2amp which slowly declines, then after about 20 seconds the fault re-appears.

Anyone any ideas of what might be wrong? Thanks!!

 

On aj27 engine upstream sensors are different type than aj26 sensors. Upstream sensor heater resistance is around 1.0ohm. Almost shorted but not.

 

Denso make a lot of different sensors so it matters exactly which were bought.

 

Buy a decent brand: I once bought a cheap O2 sensor that claimed it would fit a 2000 XJ8. Well it fit but I kept getting that 0174 warning. It was only after replacing it with a Denso, (or Bosch), sensor, the problems went away.