You can identify problems in a system BEFORE OBDII can set a code. You will know something is starting to fail, or getting weaker, before you have a light on the dash (and the resulting diagnostics which MAY be more pinpoint.  MAY, because a trouble code typically takes you down a diagnostic trouble tree to check out all of a system…. the same trouble tree you would use if you did not have the code. Most codes do not say “Replace xxxxx”…they say you have a misfire on a particular cylinder (or group of cylinders) – and then leads you differently by each manufacturer to determine whether it is coil / plug / wire / injector / compression, intake leak or whatever – in the order of most likely).  The fact is, without a code – OBDII is helpless.  If you already have a code – that’s where you start.  If you do NOT have a check engine light / code set – then Exhaust Diagnostics is the only way to identify a potential fault.Because you can identify problems before a code is set – you are in better position to offer services before additional damage is done.  Waiting for a code to set, and the resulting drive time before getting it serviced can cause damage to things like the catalytic converter, O2 sensors and so forth.  Not to mention poorer fuel economy and performance.   The fact is, using Exhaust Diagnostics on every car is the best way to sell preventative maintenance services.  OBDII readers are the quick way to deal with cars that ALREADY HAVE A KNOWN PROBLEM.