Just sharing my experience on P0661 with fellow members here.
Not sure if this has been mentioned before, since I didn't read all the p0661 thread.

I've had p0661 pop up from time to time, but only under one circumstances.
After disconnected battery when ECU relearning fuel trim.
It'll pop up only if I stall at low rpm, so if I don't stall, it won't pop up.
And after few drive cycle, it's gone.
Never test the solenoid, so don't quote me on it, but it's been like this for more than 3 years. No performance issue or what so ever.
Only pops up if I stall when ECU relearn fuel trim.

Mods I have related to ignition/drivetrain.

MS CAI
Under drive pullies
Oil catch can
Antigravity battery, x-10
Apexi ground kit
MSD ignition wires

My guess is that, probably before the motor stall, while at 750- rpm, the alternator wasn't sending enough current to the solenoid, that's why it triggers the code. Maybe? Or is it ECU related? Just shooting in the dark here. lol

Just sharing my experience here, not looking for solution, any input welcome, including flames. lol

 

replace it and u will never see the code again, no matter how low/relearn fuel trim/etc. trust me.

 

there's already numerous threads on the subject, if you ever get a P0661 code you can almost guarantee it is a sign that the solenoid is failing.

in those same threads are the part #, which can also be found new for $30 with a little more searching.

 

So, after replacing the solenoid like nycgp suggested..p0661 is gone, but ABS and TSC/DSC lights are on...lol typical rx8.

Brake light works, never replace them.
Fuses are good.
Tried turn all the way left and right, didn't work.
One post Charles from BHR mentioned to check the MAF and connector....
Interest point, why is that tho? I'll check it later today any how...

But never than less, typical rx8, one wave (trouble) after another. lol

 

probably just erroneous. did the battery get low and require a jump? also if you plug in some scanners with the key already on it may set random bus codes in various control units, they will clear out once a few drive cycles have gone through.

 

The battery is fine, didn't have to jump start, and I don't have any modified electronics in the car. Wel, only oil and water gauges.
Hopefully this goes away soon. :P