Built in A/F meter on the S4s (GL pin on the diagnostic connector)
 

I was bored tonight, so I built a diagnostic LED tester as described at http://fc3spro.com/TECH/HOWTO/ERRORC...KI/zerror.html

After complaining about the O2 sensor slightly until the engine warmed up, the lights stopped indicating anything, which was good. But I went for a drive to see blinking lights, and I wanted some. So, I hooked it up to the GL (Green Lamp) connection.

The behavior of the LED readout is very simple. On = rich, off = lean.

On my car, it seemed to be working properly.

Idle: Rich
Hard acceleration: Rich, occasionally flashing to lean
High RPM (above 3000 or so): Rich
Closed throttle above 1500 RPM: Lean
Cruising: Blinking at 1-3 hz (closed loop mode)

It's not the most useful gauge on the planet, and it's useless for any sort of tuning, but the data is there, all you have to do is wire a LED to it (with the proper resistors). It will at least let you see if the ECU is getting into closed loop mode at cruise, and it will show a severe lean condition on acceleration (though other indicators, like detonation, are probably going to appear first, at least on a turbo).

I searched, and only found one thread that referenced this, and it was buried within the thread. Hopefully someone finds this of use (and at the very least, it's another blinking light you can add that displays some amount of useful information).

-=Russ=-

congratulations, your o2 sensor works!

mine works too.. :-D

This is an easy way to determine whether the ECU is going into closed loop, which tells you if the O2 sensor is healthy. Under cruise conditions the light should flash rapidly as the ECU cycles rich-lean-rich lean. Slow flashing or no flashing means the sensor should be replaced.

Unfortunately that's about all it's useful for.

What would "rapid" vs "slow" be, in this case?

I know it's not a terribly useful plug, but it's still a meaningful blinking light, and I'm willing to bet a lot of people with "why does my fuel economy suck?" threads could gain a bit of insight from this plug (both ECU trouble codes and closed loop confirmation or not).

-=Russ=-

I totally agree. It's a simple and quick way to check the O2 sensor's health, which is exactly what Mazda put it in for. People should use these little tricks more often. I just didn't want your thread title (Built-in A/F meter) to give anyone the wrong idea. ;)

As for closed loop cycle speed, the FSM says 8-10 flashes per second at 1500-2000rpm. The sensor's reaction time slows as crap builds up on it's surface over time, so the flash rate will be slower on an old sensor.