Ok, so I thought I originally had a faulty 02 sensor code, but it turns out my mechanic had written a few numbers wrong, and it came out to being a P0178 code - Fuel Composition Sensor Circuit Low Input. I only have a general idea of what this could mean.

Could anyone possibly tell me what exactly it means and how it affects the car? Also, if it is a bad thing that will get worse if I keep driving with it, and how easy/hard/expensive it is to fix.

Thanks.