Engine light on - '91 940 B204E

312 = Knock signal from ignition system absent 213 = Throttle position switch signal faulty at full load 113 = Fuel supply too weak/rich

If I reset the error, after some kilometers it will appear with same response (without distinction between petrol or LPG).

I tried to replace knock sensor with another used and checked with oscilloscope, but nothing changes. I never push full down the throttle control (error 213???)

Checking ingition system it says "111"

The car run ok, only little difficults to start if warm engine was previously turned off with LPG supply. Perhaps it is only a little rich at idle.

How I can try to check? Mass air flow meter?

Thanks.