seen this same problem before, car runs fine for up to a month, then LOS. this was going on for 6 months or so, driving the route guy crazy. I came to troubleshoot the car, i would watch the car run all day, not a hiccup. I did not believe it was the valve, so started to change various parts, disable some etc. finally changed out the main cpu board. still the problem persisted. finally ran out of things to change so we replaced the valve and the problem went away. I would have bet the house that the valve was not the problem, as i could run it all day and never see a valve problem. I would have lost. replace the valve