Took my car (98 Mercedes C280) in to get a smog certification (CA) and it passed the emission test with no problems, but engine light indicated the following:  misfire detected (cylinder 6); misfire detected (Cylinder 5); misfire detected (Cylinder4); sensor heater fault; and fuel trim malfunction.  Have been using a cheaper grade fuel, as prices are so high and car has been running OK.  Could this be causing the problems?  Can just see me getting ripped off to fix this, are these major problems? 

yes it can check your owners man. see whats the fuel octane surpose to be if its 97 octane then its burns in the engine different than lets say 92 octane so use what the manufactor says