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Discussion Starter · #1 ·
got a CEL the other day, so i pulled out my trusty OBD-II reader. i got a P0420 "Catalytic converter efficiency below threshold". no other codes were present. if either O2 sensor was bad, i should have gotten an additional code (e.g. "Lack of Switching"). besides, i have replaced both O2 sensors, so i don't think either of them is the problem.

popular opinion on this forum appears to be that P0420 means bad MAIN cat. now, i know alot of us have had bad main cats; mine has been replaced before, and now i'm 3K away from losing coverage under the 80K emissions warranty. but before i go fight the dealership to replace my main cat, i want to check this logic:

the downstream O2 sensor is between the pre-cat and the main cat. because it is positioned ahead of the main cat, it is useless in assessing the operation of the main cat. whatever the O2 sensor reports concerns emissions before entering the main cat. so if it reports a bad cat, the finger should be pointed to the pre-cat instead of the main, right? but gutting my cat shouldn't fix the problem, unless there is a blockage, right?

secondly, the ford manual suggests i check for exhaust leaks, fuel pressure, and intake pressure. i was wondering if anybody suggests alternative/additional actions. the car has 77K on it with original plugs, wires, and sensors (e.g. EGR, DPFE, IMRC). and no perf mods, yet.
 

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As in my PM, Gary- I believe this may be a lower O2 sensor failure. 60k is about the limit of reliable life on many parts on these engines (belts, water pump, plugs, wires). The main cat is not suspect in this case because the car has no way to test its efficiency. The code you're receiving says that the precats aren't registering with the lower O2 sensors as "properly efficient". Trouble is, which one?! Bank 1 is the rear, bank 2 is the front, but your error code gleans nothing as to the offending bank. Either sensor is a snap to replace. Trouble is the cost.
 

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Being a lower O2 sensor, couldnt you just put MIL eliminators on both, and resolve the problem without having to actually pay very much. I mean, the upper O2s read the A/F ratios right? So, cant you just use MIL eliminators. Isnt that what is done when you buy headers?
 

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Discussion Starter · #4 ·
Originally posted by: DanG
As in my PM, Gary- I believe this may be a lower O2 sensor failure. The main cat is not suspect in this case because the car has no way to test its efficiency. The code you're receiving says that the precats aren't registering with the lower O2 sensors as "properly efficient". Either sensor is a snap to replace. Trouble is the cost.
precisely! that's why i don't want to order $200 in O2 sensors unless i have narrowed it down. :) the Ford manual is written like the O2 sensors aren't even suspect unless there is a DTC in addition to the general P0420.

Originally posted by: tropiccougar01
Being a lower O2 sensor, couldnt you just put MIL eliminators on both, and resolve the problem without having to actually pay very much. I mean, the upper O2s read the A/F ratios right? So, cant you just use MIL eliminators. Isnt that what is done when you buy headers?
yes, i could replace the lower O2 sensors with "MIL-eliminators" to get the CEL to disappear, but that wouldn't solve the actual problem. for instance, if my fuel pressure was higher than average, i would still be driving around like that, eventually ruining all my cats.
 
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