My guess is that the intake has thrown your MAF out of calibration. The MAF is what tells the ECU how much air is going into the engine, and this measurement is used to calculate how much fuel to squirt into the combustion chamber.
AEM/Mazdaspeed put the MAF sensor after a bend in their intake design and use too large of a diameter pipe. The too-large pipe causes the MAF to report overall lower air flow. This leans out the air/fuel ratio since the engine is getting more air than the MAF says to expect. The ECU will see this, though, since it monitors the fuel ratios via an O2 sensor, and it will quickly try to compensate by learning new fuel trims (fuel adjustments). Unfortunately, the required corrections aren't linear, and the ECU is not capable of making elaborate corrections. Furthermore, the bend in the intake pipe causes turbulence in the incoming air, making things even less predictable. When the ECU sees its own fuel trims off by large margins for a certain period of time, it will throw a CEL. This most frequently happens in stop-and-go traffic when the ECU is constantly trying to optimize its fuel trims for idling, then driving, then idling again.
Unfortunately, AEM/Mazdaspeed intakes threw the same CEL on other Mazdas, too, for all the same reasons. It's a shame as it really sours the Mazdaspeed brand. If this intake is throwing "lean" codes, then that means it can lose its Carb certification, too- but as a Canadian, I doubt you care.
Your options are to:
- Reset your ECU and hope it doesn't happen again.
- Buy an air/fuel tuner and lots of dyno time to recalibrate your intake. This is expensive and difficult.
- Sell the intake and buy a CPE Xcel intake. This intake will keeps your MAF calibrated to stock specs.
Since the engine is tuned so conservatively from the factory, your engine probably isn't at any real risk- yet. It will be if you add on other modifications that continue leaning out the fuel mixture, though- a high-compression, high-boost car needs accurate fuel metering. If you do keep the intake, remember to keep checking future CEL's with a scan tool- you'll never know if it's a too lean code or something much more serious.
For more reading, there's a discussion on this why this intake throws off your fuel trims here:
http://www.mazda3forums.com/index.php?topic=70133.msg1251416#msg1251416
Anyway, it sucks that Mazdaspeed has released yet another intake that throws CELs. I tried warning some guys in that other thread but got a lot of flack for it. All I wanted was for owners to check their fuel trims.
