I would recommend all 451 owners to replace with an air filter be it above or ebay vendor at least every two years.
Reason, all diesel engines come with a supercharger (Cdi) and most petrol versions with a Turbo (petrol), and the last thing you want to do is starve the engine of air., if you have a very dirty air filter.
My own Cdi at the time had a engine management light come up and car went into limp mode (gets you home slowly, with lack of acceleration, cannot get up hills easily, i,e hellish stupid limp mode), and I checked the code with a OBD code reader (£10 or $15 on Ebay) and it read P0299,
The P0299 DTC code refers to a condition where the PCM/ECM (powertrain/engine control module) has detected that the bank "A" or single turbocharger or supercharger is not providing a normal amount of boost (pressure).
So the first and cheapest solution is replace air filter, and so I reset the code using the OBD code reader, and replaced with new air filter, and all was fine, as filter was very dirty and partially blocked
Other but more expensive faults could be cracked air hose, faulty supercharger or turbo, but replacing the blocked dirty air filter is the first thing on the list.
So the moral is change that air filter more often, if you want to avoid triggering the engine management light and end up in limp mode
Last edited by Col Bolla; 05-31-2018 at 12:50 PM.