Citroen 1.6 2003 Xsara Picasso-Diagnostic check says PCM needs replacing??

Hi, I have a 2003 Picasso, 1.6. Orange dash light came on, meaning engine fault. I had the diagnsotic test done and the code that came up was a new PCM required. The guy at the garage turned the engine warning light off and said see how it goes, if the problem is still persisting then the light will come back on. Has anyone else experienced this? Apparently if it does need a new PCM its going to be very expensive, what happens if I just leave the light on? Car is ruuning beautifully at minute! Thank you.