[Elwarpo]

The wife's WJ (4.0L 2004) threw P0135 and P0155
P0135 (M) - O2 Sensor Heater Circuit (Bank 1 Sensor 1) - Oxygen sensor heater element malfunction.
P0155 (M) - O2 Sensor Heater Circuit (Bank 2 Sensor 1) - Oxygen sensor heater element malfunction.

The question I have is which sensors are these (the FSM calls them 1/1 1/2 2/1 2/2) and how do you replace the heater?

 

[f9k9]

That's your 2 top ones just before the precats, as I call them. Check for frayed wiring and your relay in the PDC under the hood. At least my '03 had the relay. You can try swapping it with the fuel pump, starter or horn relay. Should be the same part number.

 

[jeepin al]

had the same codes come up on my wife's (soon to be my DD) 2004 Grand. I changed out the O2 sensors and it went away. Seems the O2 sensors have heaters in them???? I first thought it was the PCM, needless to say it was just the sensors. After 150K I was surprised how easy they came out!

NAPA carries them, the different part numbers they have are for the way the plugs and wire lengths differences, the actual sensor I understand is the same on all four.