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Reduced Performance Mode


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My "S" has decided to go into 'reduced performance' mode although it still seems to perform better than lots of things on the road!  However, can anyone tell me if loss of air conditioning is also part of the 'reduced performance' mode or do I have an additional problem?

I was wondering whether the ECU also removes the AC compressor to reduce engine load.



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I think that you have two separate issues here, both of which I have suffered. The loss of power mode can often be cleared by waiting say 10 secondso after turning on the ignition but before starting the engine. This appears to allow a reset.

The AC system is susceptible to leaks in older cars. Refilling the system gives a temporary fix but is now illegal if you knowingly refill a leaky system. 

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Thanks for the response, Simon - I wondered about this and I think you are right.

Although the car went into low performance mode on heavy acceleration, indicating perhaps an egr piston problem.  I was able to reset a few times by restarting but now the amber engine light is on permanently.  However, the first time it went into low performance mode, I also lost air con at the same time and the car got very hot inside. (engine temp was normal).

 I suppose the answer is to sort the engine problem first then see what happens to the air con.

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I also have had the same problem.  It first developed after I had the Jaguar dealer clean the injectors at 50,000 miles.  The fault codes that showed up were PO442 (evaporative emission system leak-cannister valve or plennum gasket leak.  PO174 Bank 2 too lean and PO171 Bank 1 too lean


I've had the above items checked 3 separate times in the last 2 years at the dealer and they have not been able to find any leaks in the above items.  There has been no actual performance reduction in the car.  The final decision from the dealer (right or wrong) was to let it be until there is enough to really evaluate what may be actual leaking, as it would be too expensive to "guess" as they put it until the strike the correct fix.  Like yours my engine light now will not go off unless it's reset.  Not a good answer, but wondering if you also had your injectors cleaned recently before the performance error showed up. P.S.  did not have any other malfunctions such as A/C.  Probably a problem completely separate from the restricted performance issue.

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  • 4 weeks later...

Sorry Clive - this is a late reply.  I only came back to post an update on the problem(s) as follows.  I added some Redex to the fuel on the assumption that my low mileage is not too good for a diesel engine and a bit of a clean might help!  It worked!  The engine light went out and stayed out.

The air conditioning problem has been diagnosed as the heater control valve so you are right, it is separate from the restricted performance issue.  Everyone is telling me that it is very difficult to change this valve and don't seem too keen to do the work. I am currently searching for a garage that can undertake the work.

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