Ok, to try and help you make a little sense of it - as you have the Diagbox info - so maybe you can use it to see where you are on this chart - but I suspect you may be a 'K' John:
You have the Delphi Injection System:

- Particle Filter Blockage Levels
| "D" Differential pressure (mbar).
"E" Volume flow rate of exhaust gases (l/h).
"g" Filter punctured.
"h" Intermediate zone.
"j" Filter overloaded.
"k" Blocked filter. |
You can read the above in Parameter Measurements in Diagbox.
Zones "g" - "k" represent the possible clogging levels of the particle filter. The objective of the engine management ECU is a continuous "h" status (whatever the vehicle mileage).
The engine management ECU issues a regeneration assistance request to return to zone "h" (depending on the driving conditions).
N.B.: Zones "g" and "k" are zones were the differential pressure is abnormal.
The engine management ECU requests activation of the regeneration assistance function in the following case: Load level of the filter in zone "j" or "k".
Blocked filter (Zone "K")
The differential pressure is continuously higher than 800 mbars (maximum counter-pressure permitted by the engine), or higher than a threshold which depends on the flow volume.
Possible fault causes :
- Regeneration assistance ineffective
- Filter clogged with cerine
- Incorrect information from the differential pressure sensor
- No additive
The engine management ECU interrupts any regeneration assistance request and signals a fault.
Backup-Mode for Reduced fuel flow
This back-up operating mode limits fuel flow, and the engine speed cannot under any circumstances exceed 2750 rpm.
The injection system switches to "reduced flow" mode when a fault is present on one of the following components :
- Clogged particle filter
- Differential pressure sensor (*)
I've just dug out the relevant bits - so if it starts going into limp home mode you know that it's started to notice!