My 2009 C5 X7 with a 2.0 HDi 138 260,000km kept throwing an ECL every time I accelerated hard and when we scanned the errors we found P0088 and P1166 which all related to high fuel pressure. I also had an "Antipollution Faulty" message that briefly appeared on the dash.
When I checked the live sensor data for Fuel Pressure I could see it spiking with a very uneven pattern and when I booted it hard to cause the error, the fuel pressure spiked at around 2000 bar.
We diagnosed this as most probably being the Fuel Pressure Regulator that is mounted with 2 screws behind the High Pressure Fuel Pump in one of the most awkward positions to get at - in fact I wasn't able to get at it so had to let my local garage do it (just 1 hr labour) and I opted for a genuine PSA replacement (ouch!) rather than my €45 Aliexpress one - I have no doubt it would have worked as it was identical but I don't want to have to do it again. I have seen other people saying that they have been able to do it themselves.
After installing it, I again ran live data and saw a high flat line with high frequency spikes +-10bar, exactly as it should be, and the pressure maxed out at 1550bar.
The car felt smoother but not as quick as before and the fuel consumption does appear to be lower - obviously it was being overfuelled with the failing regulator.
I hope this helps someone in the future as the Antipollution error message is extremely confusing
