Replaced the O2 sensor and hooked the muffler back up.
Found that the car DOES have a little button and an LED in the battery compartment. Before pressing button, X11 pin 3 read 12.5V

Instructions say press button for two seconds and it will flash once indicating no trouble codes (Check Engine Light is not on). Mine did not flash, went to steady on, brighter when button held than when release.

Was seeing key on - engine not running steady state voltage of 6.6V - so about 50%. Not clear what that means.

Started car and let the O2 sensor warm up for a minute. Saw the duty cycle moving around, between 55% and 34%, roughly. Gave the tiniest twist counterclockwise to the lambda allen screw, brought it closer to averaging 50%.

Revved the car up to about 3500, saw it seem to be more clustered around 50%. Tried to rev it higher, before it would not exceed 4K in neutral. It sort of hung and that point, then broke through and revved up to redline!

I suspect I might have blown out a clogged injector?

Took it for an aggressive test drive. Not perfect, but running better than before. I have a feeling that putting some miles on it will clean out the injectors. Or at least a hope.

It will rev up past 4000, but it is not making much power, I would expect to get pushed into the seat, but really it is watching the needle slowly climb in 3rd gear.

Any suggestions on the next thing to check? I suppose I should replace the injectors, but am holding off on that.