The EHA relies entirely on O2 sensor output, so before even considering touching that thing other than replacing the 2 o-rings for leaks, replace the O2 sensor. To save $60 splice in 1990 Mustang GT sensor. Identical unit minus the connector.

Never taken one apart, but could just be a combination of wear on the magnet from ethanol fuel and deposits over time. Only option is replacement, and its not always drop in and go since over the years the mixture is adjusted (wrongly) to compensate for wear. Once the new one is in, everything is thrown out of whack again.

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