My 2001 XJ8 3.2l recently threw these codes but as MOT was due put her in to be tested yesterday irrespective as I wanted to know what else needed doing, and as expected emissions through the roof (binding FLHS brake calliper and jammed back middle seat belt also identified).

I like to do one thing at a time and progressively eliminate problems, so am addressing as follows:

P01647-Heated O2 Sensor 1
Today replaced LHS upstream 4 wire Lamba sensor (took SNG Barratts advice and fitted cheaper version used in X type which they now stock and supply as standard with no problems).
Reset Codes and this seems to be working fine with no repeat or pending P01647, but only done about 30 miles/4 starts.

P0128-Coolant Temp below Thermostat Required Temp
This code still showing as pending after above reset.
I understand that P0128 is time dependant and if normal operating temp not reached in about 15 minutes will cause A/F ratio to revert to fuel rich (closed loop start up mode?) and as my engine coolant and engine cooling fan seem fine believe most likely causes to be defective:

• Thermostat
• Engine Coolant Temperature Sensor
• Intake Air Temperature Sensor

Replacing the thermostat seems the most likely starting point, followed by ECTS and if necessary IATS.



(Engine temp perhaps a little slow getting there but reaches normal mid range and heater seems OK but could be hotter.)

Any advice on above, including location and checking of sensors to avoid unnecessary purchase, anything I may have missed and anything else to watch out for when replacing them would be most gratefully received.

My thanks in anticipation.
John

 

It looks like you have a good plan. While changing the thermostat it would be wise to replace the tower, connecting pipe and hoses. The plastic parts tend to get brittle and hoses don't last forever, either.

 

Mine threw that code due to lack of the sealing rubber ring around the thermostat. Water leaks around it and the engine takes forever to warm up. Could also be the thermostat itself.