I have an XK8 1999 with the following IDS codes

Engine control - P0336, P1646
Instrument pack U1260, U1261
Transmission control P1798
Climate control - request cannot be processed due to a bad specification or missing parameters
Restraint control module - communication with module has been lost.

I am guessing the P0336 and the P1798 are the key ones to go at initially in relation to limp mode and poor ruining.

Any thoughts ?

 

Before going any further, use a voltmeter to check the available battery voltage. Many random DTCs and instrument cluster warnings are the result of low voltage.

With the ignition in the OFF position for at least 30 minutes, use a voltmeter across the battery terminals to determine the available voltage. If the voltmeter shows less than 12.6 volts, charge the battery for five hours at 2 to 5 amps and retest. If the voltage is still below 12.6 volts, the battery is suspect and should be replaced.

 

Was the engine running properly before?
Did this suddenly occur?

P0336
CKPS reluctor (on drive plate) foreign
matter / damaged teeth
CKPS sensing circuit intermittent open
circuit, short circuit to ground, short
circuit to high voltage
CKPS failure

P1646
HO2S heater failure
HO2S sensing circuit short circuit to
ground or high voltage
HO2S sensing circuit open circuit
ECM failure