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I've had CEL code p0420 along with P0171 and i figured my catalytic converter is bad, so I took it off in the hopes of an easy replacement. It has been everything but that.

First, when disconnecting the pre cat O2 sensor I noticed that it was almost loose because it just came off as if it weren't screwed in tight.

Then I ended up buying a cat direct fit from autozone and it didnt have a hole for the post cat O2 sensor. So I returned it.

Then I bought a cat from O`Reilly Checkers auto parts and theirs did have a hole but it was too large for the post cat O2 sensor to screw in. Looked for an adapter everywhere and couldnt find one.

I guess my questions are this. Can a loose O2 sensor cause CEL code P0420? Did anyone else run into this problem with the post cat hole being too big for O2 sensor?

Thanks.
 

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Unless there was an exhaust leak due to the sensor being loose, then it's unlikely that was setting your P0420 (cat efficiency) code... however, whatever is causing your P0171 (engine running lean) COULD potentially cause the P0420 code to set.

A vacuum leak, dirty MAF, or a lean-biased oxygen sensor can cause the sensors to read wrong, fooling the computer into thinking the converter isn't working properly.

As for your rear oxygen sensor hole problem, I have only ever seen two sizes of Oxygen sensor.. a large sensor using a 7/8" or 22mm wrench and a small sensor that uses a (if i recall correctly) 9/16" or 14mm wrench. The only vehicle I've ever seen that uses the latter is early OBDII Nissans.

However, I don't recall EVER working on a 1996 Escort (I've worked on thousands, and don't ever recall seeing a 2nd Gen that was OBDII) and I didn't even know any existed until recently.... so I suppose it's possible that the 1996 uses a small sensor in the rear.

Now, you have two options here... purchase a universal larger oxygen sensor and wire it to the connector of the old sensor, or take both converter assemblies to an exhaust shop and have them cut/weld the old fitting into the new exhaust pipe...

OR you can put the old converter back on and chase the cause of your P0171 code and hope that was causing the P0420 as well.
 

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A bad post catalytic converter 02 sensor could be the cause of your PO420. Just a chance. I think I'd try a new 02 sensor before replacing the converter. You can buy universal 02 sensors off of ebay for about $20. You just cut the plug off your old sensor and spice it onto the new sensor.
 

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ive got a dead o2 in my car and the only obd2 code its reading is egr low flo..cuz i disconnected it due to issues so i dont know why it would throw that code at u
 

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I find it vary strange that you would get no code with a bad o2 sensor. It is designed to throw a code when anything is wrong with the o2.
 

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me neither lol but for the longest time it did i cleared it and nothing since silex says its gone closed loop so its ignoring the second and only reading the 1st. is this possible?
 

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i have a `96 1.9 lx and they are the transition year to obd2.the `96 has 2,18mm, o2 sensors,one upstream,one down.it sounds to me like the parts store gave you a cat. with an egr port downstream by mistake instead of the 2nd o2 port.that`s why the port was too large for your o2 sensor.i went through this with autopartswarehouse.com just a few weeks ago,when they sent me a cat. w/ an egr port at the exit flange after i explicictly asked them if it had the o2 and not the egr.they paid for the return and sent me the correct cat. in like three days w/ free shipping.
if you find that you do need to replace the cat. autopartswarehouse .com has the cheapest price i could find anywhere @ around $175 w/ free ship. it`s eastern cat.:#30371.it`s a high flow,direct replacement w/ both o2 ports for the `96 1.9 lx
hope this helps. by the way i called their tech line and they gave me 10% off for talking with a real live person,which knocked the price down to like $160.autozone wanted $260.
 
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