My AMM was replaced a few months ago, but wasn't adjusted at all. It was a used AMM from ebay. I passed on HC and NO, but failed the CO(4.53 with a limit of 1.16). Could the AMM be that far out of adjustment, or is it likely something else? I am going to try the LED test today. Just wondered if anyone knew the range of adjustment in turning the screw. Thank you.

Just put a rebuilt AMM in my 240, had to turn the screw about 4 turns to get it in using the O2 sensor and a digital voltmeter.

I had to adjust my junkyard AMM 10 turns to get it in the right range. I used a digital multimeter on the O2 sensor test point and adjusted the AMM until the voltage fluctuated from between about 0.25 V and 0.80 V and spent about an equal time at each reading. I re-did my E-test and got zero CO.

Ron
87 745

You do this while the car is idling at operating temperature? The same test to test the oxygen sensor in the Bentley manual?

if your AMM is with CO adjusting screw, yes. it has a tamperproof seal, which must be removed first. remove it, disconnect AMM connector (ignition OFF), and using ohmmeter, check resistance between AMM legs 1 and 6. basic setting must be between 375 and 400 ohms. good luck!
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