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Okay, I'm stumped sorta. I went and got my 1991 S-10 Blazer 4.3l smogged and failed badly.

Timing set at 13 BTDC. I know that should be at 0.

15mph/1447prm = 6.3% CO2, 1.3% O2, 624 ppm HC (max is 121), 12.45% CO (max is .70), 114 ppm (max is 1006).

25mph/1402 = 6.2% CO2, 1.3% O2, 725 ppm HC (max is 101), 12.63% CO (max is .90), 220 ppm NO (max is 866).

I've pulled the following codes.

42 – Bypass or Electronic Spark Timing circuit - The EST signal did not change when the ECM applied bypass voltage to the ignition module.

43 – Knock Sensor Ciruit - Knock sensor signal was either above 3.5 volts or below 1.5 volts for at least 20 seconds.

44 - Oxygen sensor or circuit – lean exhaust detected. - Oxygen sensor voltage was under 0.25 volts for up to 4 1/2 minutes of closed loop operation.

Now, I think correcting the timing will fix codes 42 and 43, and replacing the O2 sensor should fix code 44.

Am I way off base, or are there any other things I should check?
 
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