What is a good scan tool to have?
Is there one out there that can help even when cel does not come on?

 

Well, I like my Diablosport Predator. It is a tuner as well as a diagnostic tool. It will read live data in real time to see what is really going on.

 

Depending on if you have mechanical skills a datalogger can be used to find faults before happening, but usually involves a tool that can do mode 6. As LxMan said you can use a tuner (most are going to that DTC scan idea) you can buy a cheap scanner for around $100 which will read codes on 96+ / OBDII / 16 pin DLC connectors (but will not read live data aka datalogger.) Depending on what your doing and what you want to scan you can spend thousands for a good all around scanner. If you are not mechanic savvy you can just go to a parts store and have it scanned and try to get help on a forum for free. (Most big chain parts stores can read codes.) Most codes that are not light emitting generally do not need to be adressed as some are pointless to fix as they do not harm / effect any drivability issue.

 

Scan Guage II is great. I have it and monitor the engine, mileage, etc. as it's running. You can change tire calibration also without effecting the trucks computer. In other words, you're just changing the display through the Scan Gauge II. Works well for me and reletively inexpensive direct from the manufacturer.
Just my 2 cents.
http://www.scangauge.com/

Later...
Brewster...