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


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


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


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....

Your Answer

Add photo

Related Questions

  • check engine light with flashing cruise control light 178 answers

    Checked codes at Auto Zone: catalyst efficiency low-bank 1 probably cause: CC failure possibly due to: (1) engine misfire or running condition (2) large vacuum leak (3) engine oil leakage into exhaus...

    2005 Subaru Forester XS


    Maintenance & Repair

  • "Check Engine" Solid - Flashing "Cruise" '05 Forester 16 answers

    I have an '05 Forester / 122k miles. Twice I've had the blinking cruise & check engine lights simultaneously appear. Both times I've managed to resolve the issue by filling the tank up with premium g...

    2005 Subaru Forester XS


    Maintenance & Repair

  • 03 subaru legacy engine trouble 2 answers

    2003 subaru legacy l wagon 2.5 Misfiring, backfiring, loss of power, engine vibration, stalling, now wont move. I have po301, 302, 420, and 457 codes. Replaced plugs, wires, fuel filter, upstream o...

    2003 Subaru Legacy 2.5 L


    Maintenance & Repair

  • Subaru with no power? 19 answers

    Does any one know what would make my 2011 Subaru w/ a 3.6R engine stall and have no power going up a grade? My OBDll reader said both cam sensors were the problem, I find this unlikely.Also I have m...

    2011 Subaru Outback 3.6R Limited


    Maintenance & Repair

  • catalyst effenciency below threshold bank 1 2 answers

    2001 Toyota Celica GT


    Maintenance & Repair


Looking for a Used Impreza in your area?

CarGurus has 8,292 nationwide Impreza listings starting at $1,995.


Search Subaru Impreza Questions

Subaru Impreza Experts

#1 FordNut
Reputation 480
#2 Nick Eidemiller
Nick Eidemiller
Reputation 420
#3 TheSubaruGuruBoston
Reputation 360
View All

Related Models For Sale

Used Subaru Impreza WRX"
43 Great Deals out of 1,348 listings starting at $2,500
Used Subaru Impreza WRX STi"
34 Great Deals out of 605 listings starting at $8,977
Used Subaru Outback"
422 Great Deals out of 30,431 listings starting at $1,295
Used Subaru Legacy"
246 Great Deals out of 18,022 listings starting at $1,300
Used Subaru Forester"
360 Great Deals out of 19,905 listings starting at $1,099
Used Honda Civic"
1,173 Great Deals out of 59,023 listings starting at $1,300
Used Mitsubishi Lancer Evolution"
20 Great Deals out of 808 listings starting at $16,995
Used Mazda MAZDA3"
482 Great Deals out of 25,014 listings starting at $1,499
Used BMW 3 Series"
1,696 Great Deals out of 39,944 listings starting at $888
Used Honda Accord"
1,308 Great Deals out of 78,748 listings starting at $1,395
Used Toyota Corolla"
1,470 Great Deals out of 67,841 listings starting at $1,295

Used Cars For Sale

2016 Subaru Impreza For Sale
5 Great Deals out of 3,373 listings starting at $16,500
2015 Subaru Impreza For Sale
34 Great Deals out of 632 listings starting at $12,999
2014 Subaru Impreza For Sale
40 Great Deals out of 765 listings starting at $11,900
2013 Subaru Impreza For Sale
70 Great Deals out of 1,175 listings starting at $9,495