Reduced Engine Performance

[CREUS]

Please help - I have a 2004 S40 T5 Geartronic with 100k kms. The car has an intermittent and re occuring problem which the dealer and Volvo SA cannot solve.

On long downhills and with the vehicle coasting downhill, with my foot completely off the accelerator pedal, the orange warning light will sometimes come on and a "Reduced Engine Performance" message appears. The car loses its power and goes into a "limp mode". If i pull off the road, switch the car off and then immediately on again, the car has its full power back however the warning light and message are still displayed. After some time driving with normal to hard accelleration, the warning light and message disappear.

This "reduced engine performance" problem is intermittent and only happens when the car is coasting downhill with my foot off the accelerator pedal. The Volvo dealer has replaced the turbo control value with 3 different ones, replaced the air flow meter, fuel pump, tried a new computer, checked all hoses and has now replaced the turbo, all to no avail.

When the dealer checks the fault codes, the codes are inconsistant sometimes showing boost pressure problems, lean mixture, amongst other codes etc.

This has been going on for six months now and i don't know what else to do. The dealer and Volvo SA say they have tried and checked everything they can.

Any suggestions?

[CREUS]

This problem still persists!
Something new that i have learnt is that if I press the brake pedal briefly as the car starts to coast downhill, this action seems to prevent the car from incurring this annoying problem of going into a "reduced engine performance" mode.

Any ideas or suggestions on possible causes will be appreciated.