Hi Here is the story I had 92 Crx with swapped stock b16 in it.the engine itself did not have a knock sensor ,and I always had cel 23,but everything was working perfect including vtec. Recently I swapped in a b20vtec(it is newly build engine with 1500 miles on it,it does not have a knock sensor ether),and now the vtec is working from time to time.When it is not working, the car would not rev past 7000.I have checked everything: oil pressure and level,vtec solenoid,ground,even checked for loosen pins in ecu,nothing seems wrong.Once when I checked vtec signal voltage from ecu,it was 11 something all the way from 0 to the red line,but just once. And just after I checked everything once again,the car started to run perfectly,but after one hour or so, vtec died again.What the hell could it possibly be? Any ideas? Thanks.