Not doubting your DIY abilities but....
What it means that you need to systematically start the problem solving tree and test everything that can generate a P300 generic Diagnostic Trouble Code. The operative word here is "Generic"!
So an experienced Automotive Technician if they physically had the vehicle would be able to rule out(but questionably) a few items and go to the less common parts to check. There is NO SHORT CUT regarding a P300 on any vehicle.

I see forum threads with DIY members give every reason for a P300 or a repeating P3000 DTC and what they did that resolved it as if there were a standard reason for the DTC itself. It is a GENERIC CODE that the ECU was not programmed to single out any specific problem area or parts.,

 

Understood. Maybe I'll have someone else look at it to see if they can come up with any other options.

 

JustaCX7, can your scan tool record individual miss fires on individual cylinders over multiple drive cycles?

Can you access live data on your scan tool? eg short & long term fuel trims? Air fuel & O2 sensor outputs?

 

I have an SCT XCAL but my friend has a data logger that we can use

 

CX-7's are common for build up of carbon on the valves & injectors.

 

I'm going to try an injector cleaner as I have read about this. But being that it has 156k miles and the previous history doesnt show a chain replacement I have begun ordering all necessary parts to perform this maintenance.

It's funny because i got this car 4 years ago and when i called mazda for a history on the vehicle they didnt want to tell me anything. I explained the timing concerns and she mentioned that she didnt see that in their records but couldnt tell me what was serviced. Funny

 

Just an FYI if you start randomly guessing you could add to the problem. Cleaning the injectors is not recommended point of concern because had it been a single injector you would have a DTC relating to that cylinder. Because it is a P300 and the chances of all the injectors a problem with no other TC is possible but the odds are very high against. If you suspect the timing components you can always check with a simple timing light.

The point I was stressing again is what a P0300 is. So unless you follow grimsreaper suggest and mine you could add to your troubles! BTW we are both Techs!

 

Greatly appreciated gentlemen. I will check back with updates.