What exactly does the sensor measure and what does the ECU do with the data?
Having dicked around with O2 sensors a fair bit on all my bikes, I can tell you that there is very little accurate information you can guarantee on how and when the O2 sensors are used (closed loop mode) on Honda motorcycle PGM_FI systems. These systems are NOT compliant with the OBD-II standard and you cannot hook up a diagnostics probe (a computer, basically) to monitor the fuel trims or whether the ECU is in closed loop or not.
In a nutshell though, when the ECU goes into closed loop mode it looks at the O2 senor signal. Each O2 sensor puts out a voltage between 0.1 and 1.0 volts - 0.45 volts or thereabouts signifies a 14.7:1 Air-To-Fuel Ratio (AFR) and this is what all closed loop ECU systems try to hit. Why do they try to hit 14.7:1? Because that's where the catalytic converter works best. Even a small amount up or down of 14.7:1, the exhaust contains more CO, or more NOx. So basically for best emissions the ECU has to hit that magical 14.7:1 ratio bang-on. Now, there is a massive amount of stuff that can change that AFR reading every few milliseconds. Engine speed, throttle position, air temperature, fuel quality, fuel octane rating, fuel temperature, etc, etc, etc. All of these are dynamically shifting all the time.
A voltage reading of 0.1 volts is somewhere down around 17.6 AFR (lean) while a voltage of 0.9 volts is somewhere around 11.78 AFR (rich).
What happens on the road is that the ECU sees an O2 sensor voltage of too high or too low and keeps adding or subtracting fuel constantly. Subtract/add, subtract/add, subtract add. If you monitor the waveform what you should see is the voltage going from 0.1 to 0.45 up and down repeatedly. If the ECU sees this, it will stop trimming. But if the waveform goes consistently under 0.45 it will add fuel and if it jumps consistently above 0.45 it will subtract fuel.
The next question is how the ECU reacts or alters the fuel. Basically it has what's called a fuel trim and it will add or subtract a percentage of fuel to the injectors for each injector pulse. Most cars' trim range will be +20 or -20 (I'm assuming percent here, but the value could refer to pulse width in milliseconds).
When does a Honda PGM-FI system go into closed loop? Don't know. Does it store the trim values permanently and store that trim to the relevant throttle/rpm position next time the bike is started? Don't know. What are the trim limits? Don't know. What is actually happening to the close loop trim when an O2 eliminator is connected? Don't know. Everyone (including me) seems to think that the closed loop mode is only at low throttle positions and cruise rpms, but where did we get our info from? I certainly can't remember.
Now here's where it gets shit on Honda motorcycle PGM-FI systems. You cannot
hook up a diagnostic probe to the ECU and find out what the trim values are. And, you cannot tell if your engine or O2 sensors are so snorked that the trim values have been exceeded! If you take ANY car with an OBD-II compliant ECU, it will flash a Check Engine Light (CEL) any time the O2 trim exceeds its bounds and you can use your diagnostic probe to actually see what the ECU is seeing!
The PGM-FI ECU has a total of about twenty error codes, and they're all to do with "sensor disconnected" or other obvious problems. If your bike EFI system messes up, your mechanic can only truly diagnose it if they are a freakin' diagnostics genius, and these guys are rare, rare, rare.
You can tell the guys who know their shit by asking to see their diagnostic probe, and if they know what a DSO is (Digital Storage Oscilloscope) and what a current clamp connected to a DSO does. If they don't have this stuff, they are not a diagnostics technician and will probably be worse at diagnosing faults than you UNLESS they are one of those freaky guys with years of experience.
Please note the information here is specific to Narrowband O2 sensors only. The voltages and cyclic behaviour of a wideband O2 sensor is totally different. Best as I can tell, all Honda bikes still use narrowband O2 sensors but many euro bikes are using wideband sensors. Powercommander offers "O2 optimisers" for many bikes now, because they either use a wideband O2 sensor or the ECU is smart enough to detect when the O2 sensor has been disconnected and bypassed using those little 330 ohm resistors we see Dynojet providing with all Honda PCIII and PCV systems. These optimisers convert the signal going from the O2 sensor so that the waveform the ECU is looking for that signifies 14.7 AFR is actually something like 13.6 AFR, and hence the ECU stops trimming when the mixture is 13.6 (better for power) because it was fooled into thinking the mixture was 14.7 AFR. You can tell these systems apart because Dynojet will tell you to leave a bunch of cells in the Powercommander software set to zero. These cells are where closed loop mode kicks in on that particular bike. By the way, these so-called O2 optimisers are just a Dynojet clone of things called "EFIE" devices; feel free to go look them up