All the above posts are guys trying to keep you from hurting yourself or others. Looking at it with a little different slant, someone is going to pay for your training. That means that either YOU pay or your customer pays. If you're willing to get the appropriate schooling and experience at no charge to the customer, then you're doing the right thing. But if you expect your customers to pay while you sit an read manuals or try to figure out a basic problem, that's just not right. Furthermore, gas is much more dangerous to both you and the homeowner if not serviced and handled properly. I'd highly recommend you attend some classes to gain some knowledge. Those classes will also let you know what tools are required to do proper service. And keep in mind, 50% of the problems will not be related to the gas unit but rather will be electrical in nature.
If YOU want change, YOU have to first change.
If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!