U0093 OBD2 specifically refers to the camshaft (cam) timing. In this case, if the cam timing is over-retarded, the engine light will be illluminated and the code will be set.
OBD 2 U0093 code and tailpipe testing are two different approaches to identify vehicles in need of repair. The OBD system looks for broken or malfunctioning emissions control components while tailpipe tests sample a vehicle's exhaust to see if it is above or below certain prescribed limits. Given the robust nature of today's emissions control components, it is entirely possible for an individual component to malfunction without leading to an immediate increase in emissions at the tailpipe. In such cases, other components (like the catalyst) can temporarily compensate for the part that is broken however, these other components can only do double duty for so long before they, too, begin to malfunction. Because of its ability to monitor individual components, OBD is able to give motorists an early warning that repairs are needed; it is because of this early warning capability that OBD will sometimes fail vehicles that would otherwise pass a tailpipe test.
The problem is caused by the variable cylinder management system switching on and off during light throttle conditions, at cruising speeds, or when driving on flat roads. The constant switching causes excessive oil consumption and the trouble codes appear when oil pressure drops to the point where it can no longer operate the VCM system.
Disconnected, dirty or fouled spark plugs are common causes for engines that won't start. Spark plugs typically need to be replaced every season or 25 hours of use. You should also check that the spark plug gap is set properly. If your spark plugs look good, problems with your ignition system can also preventing a spark. These can range from a faulty spark plug lead, shorted kill switch or flywheel key damage.