Couple of questions

Well guys, with your and others' help I am finally starting to narrow down my "Check Engine" light issues. I have treated all my connections (with deoxit - better than Stabilant), replaced the phase sensor and the relays, and now I am down to codes re the rev sensors and the compensation valve. I couldn't get at those to treat the connectors, so now I am going to try to go in from the access panel by the rear seats, (it's a Mondial T), and up from below.

Question: Does anybody have a good source for cheap replacement Hall sensors for the rev counters?

Question: Other than bad connections, are there any known failure modes for the compensation valve. Maintenance suggestions?