'92 S10 Blazer service engine light

1992 S10 Blazer 4x4 4.3 liter just over 160,000 miles. The service engine light comes on just after start up and remains lit until the engine warms up, the colder it is outside the longer it
remains lit. I had it checked and the only code was a 43 (knock sensor?) The voltage at the sensor was 3.9 vdc which the mechanic said was a little low but that it should be alright. I also replaced the knock sensor just to be on the safe side. No one seems to know why the problem seems to be temperature sensitive, or what to do about it. Any comments, hints or suggestions would be a big help.
Thanks folks, Hope you all had a great Thanksgiving. Where applicable.

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