Engine Diagnostic Light

I have a 2004 (54 Plate) Carisma 1.9 DID, recently the Airflow Mass Sensor (Part No. M887504) went down, although the degeneration was gradual until finally it lacked any power at all when the engine was under load. As I could not afford the =A3405 + VAT that Mitsubishi wanted, I managed to locate a second hand part that seemed to cure the problem. I say seemed because after just 2 days of running, when climbing a gradient form a cold start the diagnostic light came back on although there was no loss of power, and once back on the level the light went out. There is obviously a reason, but what is the problem? Has anyone else expienced something similar. Any help please!