CEL on and no code

[Nordic92]

I got the CEL on but cannot get codes
Few days ago, doing nothing, the CEL turned off; I was got a code (34 MAF sensor)
Run for about 20 minutes, then the CEL turned on again, cannot get codes anymore (and engine running bad again too)
Question: why can I get any code while the CEL is on? would it be the ECU, a bad ground?
I disconnected the MAF, should be running good but still not - got another MAF, still running bad - so I suspect the ECU is in a bad loop or something (visual inspection of the ECU: perfect, nothing bad, like new)

thanks,
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Sidekick 92, 4dr, 16v, 5 sp - +200,000KM