I own a 96 Bronco 351. I ran a diagnostic scanner and came up with 5 codes all refering to O2 SENSOR HEATER CIRCUIT MALFUNCTION for all 3 oxygen sensors and SYSTEM TOO LEAN for front and back sensors. I can't seem to pinpoint the problem. I have replaced all 3 O2 sensors,checked all the wiring and connections to the O2 sensors, checked all fuses and sparkplugs, replaced corroded + battery cable
($130 special order from dealership),and finally the PCV valve was replaced and still no luck clearing those codes. After erasing these codes they only comeback after 3 start-ups, and won't pop up after 1 start-up and 30 min. of driving. If anyone could give me some tips, hints, or info. concerning this it would be appreciated greatly. Thanks

Greatly Appreciated,
Robert

 

See if you have a grey/yellow wire and a black wire at the oxygen sensor. The grey/yellow is the 12 volt feed for the heater on the sensor. The blk is the ground. Check across these two wires. If no 12 volt, check from the grey/yellow to a shiny metal place that is known to be a good ground. If you have 12 volt, you know there must be a bad ground where the blk wire connects. If you still don't have 12 volts, the diagram shows a fusible link feeding this circuit. The color of the wire is shown to be grey/yellow on both sides of the fusible link. I can't seem to determine where this fusible link would be located, but a good guess would be off the EEC power relay. See if you have a relay box on the fender somewhere that has a bunch of these fusible links coming out of it. It's supposed to be a blue 20 guage fusible link. It may be burnt open.

 

 

Broncoman,

Did you ever figure this out? I have the same Bronco with the same problem.

Thanks,

Martin

 

WOW...very old thread, no matter Welcome to FTE.

 

It's driving me crazy.

Thanks for the welcome!

-Martin

 

Post a new thread, tell your problems and we'll see what pops up.