Thanks for that Spelunker You've saved me a job.
I had a similar problem, sometimes the sensors worked fine sometimes continuous tone even though all sensors were clicking correctly.
I couldn't figure out whether to tackle the front or back first.
Just checked my sensors and found one on the front and one on the rear with surface paint bubbling.
Scrapped bubbling paint off both and flattened with emery and low and behold the sensors are now work correctly.
Hand painted them in and they now look as good as new. 
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