A week ago I was doing a long journey (180 miles) at night and travelling quite quickly. I passed two cars up a hill and I expect I 'floored it' for 30 secs or so. All was well for about 2 minutes after that then the car lost power and the dreaded 'Reduced Engine Performance' warning came up. I motored on (slowly) for about 5 minues then stopped in a lay-by for another couple of minutes with the engine off.

Started up and was still under REP. Moved off and about 10 secs later the warning went off and everything went back to normal. A week has passed and the same journey completed without repetition of REP. There are no fault codes stored or pending on either module. Would be most grateful if some one could tell me what the likely cause of the REP was and why no fault codes were stored. From the symptoms, it feels like a temperature related issue, but I really have no idea!

were you low on deisel? It could be that the car went into limp mode which can happen when fuel is low.

Plenty of fuel, probably half a tank. Can't think it was that. Thanks for the suggestion though.

What fault code reader did you used?
Because there are various DTCs: permanent, pending, historical, etc.
If the tester is able to see only one category of them (generally the permanent ones), then you won't be able to properly diagnose the fault.

Hello Alex. I have a Gendan EngineCheck Pro v3.0. I know it reads stored and pending codes (at least it says it does!). It also enables viewing of live data and apperently, freeze frame data. I would be very interested if you think it is not 'seeing' some codes. Many thanks.