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I have a 1996 J150gledb that has been giving an intermittent check engine light. I have checked the vacuum switch function , it opens around 5.5" of vacuum. Occasionally the light will come on immediately after the self test sequence wit the key on nit running. The last time this happened I unplugged the switch and the light remained on. The horn sounds like it should during initiat ion and when it goes into alarm with motor running. Any ideas what to check next?