I've not heard of any "unreadable" powertrain codes. All powertrain codes should be readable, but the tool used to read them may not be able to provide the definition. Here's why:
When the standard for powertrain codes was set up, it was realized that manufacturers may wish to test things other than what was standardized. So a differentiation was made. Codes P0xxx indicate a standardized code. They indicate the same thing on all vehicles. Codes P1xxx and P2xxx are manufacturer specific. All manufacturers use them. And all appropriate OBD-II code readers should be able to read them (although not necessarily give their definition).
At this point, I'm interested in what the specific code was (not what anyone told you about what it meant). There is no code (except P0000 which appears to indicate all good on some vehicles) that indicates nothing was wrong. Someone here is not giving you the full story. Every person who checked the car should have been able to give you the code number, whether the reading was free or not. If you have the code number, I'll be happy to tell you what it means. Trust me, it means *something.* Of course, that something could be the same thing your other oxygen sensor code indicated.
As far as the dealer service department, it's not generally in their interest to scan you car for free. The service department makes money off the time they spend on your car and the parts used to repair your car. If you're not willing to pay .5 to 1.0 hours labor in most cases for the reading of the code and proper diagnostics, then you're probably also not thgat interested having the dealer repair your car.
Parts stores, on the other hand, want to sell parts. Of course, they typically know nothing about actual diagnosis and the fact that most trouble codes can be caused by a variety of things which are in some cases, not necessarily obviously related to the code. But they provide the code-reading service because it helps them sell parts.
Bottom line, it's all about business. And the issues you're having are with the businesses, not with the car itself.
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