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I have a 96 Legacy AWD 5-spd 2.2 with 160K miles, and with the aid of a scanner found trouble code P0420 "Catalyst system efficiency below threshold (CAT)." The factory manual says if there are no other trouble codes showing (I have no other codes showing), to (1) check for leaks in the exhaust system (I didn't feel any exhaust leaks by floating my hand along the exhaust system), or (2) remove the front and rear catalysts and check for damage at the face of the catalysts.



Question 1:

My main question is, if I remove the catalysts to visually inspect them (and it looks fairly straightforward to unbolt the section and reinstall it with new gaskets), would "damage" at the faces be fairly obvious, or is damage or degradation subtle?


Question 2:

How many miles should catalysts typically last (most of my miles are highway with Chevron regular gas)? I've had two other cars that sold with over 300K and original good catalysts.



Other background info:

This model has a front and rear catalyst, with two oxygen sensors, one in the front of the front catalyst and one in the rear of the front catalyst. My guess is that with oxygen sensors on both ends of the front catalyst, that the OBD-II system could detect specific performance deficiencies occuring at the front catalyst. Since there are no sensors downstream of the front catalyst, the only performance deficiency that the system could detect occuring from the rear catalyst would be obstruction or reduced (or increased) flow, which I would think would be obvious visually.


The only problem I noticed from the outside was the support bracket between the catalysts that secures them to the transmission extension housing was broken in half, although it looks as if this has allowed the rear oxygen sensor to hit an adjacent bracket as it has a dent in it.


I will try to test the oxygen sensor response with my scanner likely as the next step along with replacement of the broken support bracket (so that's one reason why my catalysts hang down nearly an extra inch), and then reset the codes before removing the catalysts, as if it's a problem inside one or both of the catalysts, the code sould reappear quite soon if not immediately.

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this error code usually occurs with aftermarket cats but can happen with OEM cats as well.


there is a simple fix for this; i quoting someone from the toronto subaru club, the wire colours are for a GC8 model impreza, but the basic fix should work.


"PO420 is the code that is given for "Catalyst system efficiency below threshold the second O2 was put in place so people would not remove their Catalyst Coverter. To put it in simple terms the O2 sensor senses how much air is passing it if too much air is passing the O2 sensor than there is something different from factury. Like removing your Catalyst or enlarging your exhaust could make your cel come on. I replaced my exhaust and it did not have any problems with the cel light coming on till 6 months. There is a fix for this. You can use a .5 watt resistor 1 meg ohm and 1.1micro ferid capacitor.

On the O2 sensor side, because the colours change on the other side of the connector you cut the white wire and connect either side of the wire to the resistor now strip some of the blue wire connect the + side of the capacitor to the blue wire where you have striped the wire now connect the other side of the capacitor to the white or where the resistor is. You can use this resistor capacitor combo for almost any car as long as you know the two signel wires for your O2 sensor. This lies to the ECU and you should not see the PO420 error code again. This error code is not bad but if you do not have a OBD II reader you do not know what the cel is. If any one has had errors PO405 to PO409 this is because you have change your up-pipe to the turbo and might of not installed the EGT sensors or you have installed the EGT sensor and your up-pipe is very big. You can install a 2.2 kilo ohm resistor, disconnect the connector on the EGT sensor which is on the up-pipe and on the wire going back to the ECU connect either side of the connector on either side of the resistor. I do not recomend this that EGT is there to tell the ECU if your engine is running ok. If you have a external EGT gauge and sensor and know how to use it then this is ok.

This fixes do not change the performance of the car in any way but the EGT is there fot the safty of your engine life to tell you if everything is ok with the engine. "


hope it helps:D

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