My 1987 744 Turbo is sucking back more gas than usual, so I started doing a number of tests related to the AMM and O2 sensor, as explained in the FAQ.

I used a digital multimeter to test the O2 sensor, probing it directly at the firewall connector while the engine was running and warm. The O2 sensor seemed to be switching at the right speed, and responded to the "vacuum leak" and "fuel pressure regulator plug" tests. I did notice that the voltage numbers seemed a little low - it never showed more than about .780 volts, certainly never hitting .9 volts at its highest. At its lowest, I think just under .1 volts. If anything, I would assume the O2 sensor was reading a little lean because of this.

Next I built the LED tester for the CO pigtail, as explained in the 700/900 FAQ, so that I could tune the AMM mixture screw. My airbox thermostat failed some unknown time ago (now shimmed permanently closed with washers), and I was concerned that all the hot air flowing in the airbox had damaged the AMM.

Oddly, the LED glowed solidly, regardless of what I did with the AMM screw. No flashing whatsoever.

Thinking that this was a sign that the ECU was seeing a solid rich signal, regardless of what the O2 sensor was doing, I figured my AMM might be trashed. I disconnected the AMM to see what would happen. The car immediately ran much rougher, and pig rich, so that I could smell gasoline. Plugged the AMM back in, and all was better. Well, that's not it, obviously the AMM is giving some kind of beneficial signal to the ECU, regardless of what signal the ECU is sending out to the CO test point.

So, Volvo gurus, why would I be getting a solid LED on the CO tester? The O2 sensor seems to be working fine, if showing a little lean, but the CO test point shows a solid LED with no flashing, which suggests the ECU is seeing an insanely rich mixture.

What's going on here?