Xc70 reduced engine performance
 

It seems a lot of XC70 drivers have seen this message crop up, and there are all kinds of suggestions as to what might be the cause. I occasionally (about once in three times) have trouble starting the engine, which then idles roughly for a few seconds and displays the REDUCED ENGINE PERFORMANCE message with the yellow warning triangle. It drives perfectly well until the light and message disappear. Recently, it's also happened on the motorway, briefly displaying for about 5 seconds, usually when going up a slight incline.

I've seen the following possible causes:

• Failing battery (although it cranks OK)
• Dirty or worn throttle body
• Failing ETM

...and the following solutions:

• Replace the battery (although I'm thinking that disconnection will reset the control software anyway)
• Clean or replace the throttle body
• Replace the ETM
• Upgrade the control software (although I understand that this increases the ETM's tolerance to faults rather than actually fixing the fault)

Does anyone know the right answer?