Diagnostic Aids
Check O2 sensor wire for contact with exhaust manifold. Check O2 wire for ground between connector and sensor. Check for fuel contamination. A small amount of water in fuel tank can be delivered to fuel injector causing Code 25 to set.
Check fuel pressure, see BASIC TESTING article. Check for exhaust leaks. An exhaust leak can cause outside air to be pulled into the exhaust past the O2 sensor. Vacuum or crankcase leaks can cause a lean condition. If Code 25 is intermittent, see TESTS W/O CODES article.