What does the diagnostic code P1646 for a 2002 jaguar X-Type?

P1646 is a fault in the heater or sensor of the upstream O2 sensor. It is the light grey plug on the firewall behind the engine and leads to the sensor unit itself on the rear bank exhaust pipe. A replacement sensor is between (UK) £65 and £120 depending on where you go. Beware of the generic 4 wire sensors as their specs are not quite up to Jaguar's. BTW. It's a b*tch to get to and you need a 22mm socket with a slot for the wires to go through.