PO420 Catalyst Below Threshold

Asked by Oct 21, 2015 at 02:26 PM about the 2007 Subaru Impreza

Question type: Maintenance & Repair

PO420 Catalyst Below Threshold Code only

This code appeared and I did a few things like cleaning the manifold and
engine, then shut off the code to see how long til it re appears(did this a
few times). It seems to come on only at high elevation, whenever driving to
the Mountains.
Next, took it to emissions testing and it passed, even after a few months of
the Check Engine Light being on.
So if the emissions pass, does that just mean i need to replace O2 Sensors
since the emissions are perfectly fine?

3 Answers

1,495

Catalyst below threshold, means that when you have the two O2 sensors, the upstream O2 takes a reading then the exhaust goes through the converter then the downstream one then takes a reading to see if your cat is doing what its suposed to be doing, If not then you need to replace the cat. Ilive in very flat land so not too sure how high elevation works, You could call the dealer to see if there is a calabration update

5,955

check this out: https://allwheeldriveauto.com/subaru-check-engine- light-and-code-p0420-explained/

3,500

Cats are sometimes covered by extended Fed warranty in PZEV States. Otherwise ignore unless you've symptoms OR an additional O2 sensor code. Richer winter fuels in new England famously cause overly-restrictive Subaru and Toyota ECUs to reject the cats' "insufficiency" routinely. Subie's escaped a recall bullet here, as their "reflash" freebie is usually unsuccessful...but if you qualify for it (check your VIN) then maybe it'll help. SOA has also been known to "participate" at 50% ($600) towards the $1200 job. Note that aftermarket cats do NOT solve the problem, but I'm hearing that there may be a pricier ($600-700) aftermarket that's OE quality. Not sure....

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