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Help on P0420 and car stalling during hot weather

I recently bought a used '07 Subaru Impreza with a manual transmission. It’s been working pretty well but there are a couple things I’ve noticed that make me wonder what’s going on. The CEL has come on a couple times due to a couple random P0456 and a few P0420. After checking the codes and resetting the DTC I noticed my car would stall frequently when I brought it out of gear for the next half hour. It was also usually on hot days. On one particularly hot day (95°+) it did it to me super frequent throughout the whole day.

I was wondering if anyone might have some insight into what was going on and what some possible solutions might be.

Additionally I want to point out that it would never die while idling. When shifting out of gear the RPMs drop and it would just get too low and stall. If I got lucky it would just get really low then stabilize in the 600-700 range.

Thanks for any help!

Ps. I started making sure my gas cap was tight and haven’t gotten any P0456 since.


The DTC P0420 points to a problem with catalytic converter.

And the DTC P0456 points to a problem with a small leak in the EVAP system.

So the tightening of the gas cap got rid of the P0456 code.

So with a ten year old car, the cat might be on its last legs.


I’ve been doing research on the P0420 and if it comes after replacing spark plugs or the 02 sensors show any signs of compromise to the cat I’ll be looking at replacing it.

The main thing I’m trying to figure out is why my car runs fine 98% of the time, but will stall when I take it out of gear after I clear a cel (total of about 5 times in 3 months with no pattern other than the connection of high temperatures).

Hey Kamaron.woods -

I used to have a WRX which was the same body style. if yours is just an impreza than we both know that you’re non turbo but the same concept applies to what i am about to say:

P0420 and P0456 are an EVAP (emissions) related codes.

I would recommend changing the gas cap for the p0456 code since what the code is related to a “small leak”. it could also be a hose or something dumb.

for p0420 : thats most likely related to your 02 sensors ( i had the same code).
Try changing ALL the oxygen sensors.

The stalling is because you reset the computer and it has to relearn… you’ll have to drive it probably 20 miles give or take for the computer to be liek OH THATS HOW WE WORK. pretty much when you feel the idle drop like its about to stall give it a little throttle, and a little clutch. it should bump the rpms back up until the car relearns. your best bet is to go to a large parking lot and do a ton of stops and go’s… so obviously you’re safe.

with my wrx and emission codes (btw… they WILL come and go depending on the weather).

if you have any other questions let me know.

I have a completely different car, an Acura RL, BUT I too have a P0420 code. It has to do with the Catalyst in the Cat not working at a 95% or greater efficiency (which to some people really doesn’t mean anything, so they ignore it), could be clogged or worse - damaged & blocked. There may be an underlying reason WHY it is failing or failed. Could be bad timing, allowing excess fuel to get dumped into the exhaust, causing excessive burning in the Cat. Who really knows? Worth a look see to figure that out.

Some people “claim” you can clean the Cat, BUT - based upon another mechanic’s research - cleaning the Cat does absolutely NOTHING. I suggest you NOT change the O2 sensors, but rather change the Cat. Put in a Walker. I’m going to do that too.

I used to own an Integra, so I’m all too experienced with pocket rockets. Allowing the car ECU to relearn how you drive is the key. So, knowing how to double-clutch should help you keep those stalls to a minimum. Does that help?

Thanks. I suspected it was probably the ecu but I had never heard of a car learning like that before.

As for the P0420, I have done a lot of research on it. Before I spend any unnecessary money I want to diagnose it better. My dad has an OBDII reader and an unlocked app that I’m planning on using over the weekend (I have a reader too, just haven’t decided which app to pay the money for).

If it’s the cat, I’ll change the cat. If it’s the sensors, I’ll change the sensors. If there’s a crack in the pipe, I’ll have the pipe fixed. There are too many options for what it could be. I just want to figure out what it is before I throw money at it.

the relearn process is very likely what causes your car to run rough after clearing your codes, but it shouldn’t cause your car to stall. Sounds as if you have another issue going on- which is what @hienz1 was talking about, that is that something else is causing your p0420 code. I would get it hooked up to a scanner and look at short and long term fuel trims and make sure the computer isn’t adding too much or too little fuel to the system to compensate for something else.

Use that unlocked app. You need to see an oscilloscope readout and other multiple readings to help you figure out just what your beastie is doing.

A few years ago we had reports here of some cars having a problem where the owners would replace the battery, only to discover afterwards they couldn’t start the car. It would crank ok, but it would never catch and run. They’d have to tow the car to the dealer, who’d “teach” it how to run again after being reset by the battery replacement. I wouldn’t have guessed just resetting the DTC’s using a scan tool would cause the computer to forget the operating parameters it has already learned, but I guess it does. It might depend on how exactly the DTC’s are reset. There may be two ways, one which makes the computer forget the learned operating parameters, and one which remembers them.

To avoid that problem, you use a 12V memory saver. Many of them simply plug into the OBD2 dlc

Saves you a lot of hassle, and they’re not even expensive. Typically far less than $100

Or, you can spend about $12.00 at the parts store for this memory saver.

Just connect a 9 volt battery, and plug it in place of a cigarette lighter, or into a power port, and it’ll save the memory in the computer for up to an hour while the battery is disconnected.

But remember to place a note on the drivers window not to open the door while it’s plugged in and the battery is disconnected.

Otherwise, if you open the door, the interior lights will kill the 9 volt battery, and the computer will lose its memory.


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relearn process on a subaru is odd… i had an 06 wrx, 5 speed and everytime you cleared the codes for EVAP related b.s. the car would have to “relearn” because the rpm’s drop so low – if you can catch it while stopped by just getting on and off the clutch and repeat… you could 75% of the time stop the stall from happening… after 10-20 miles you’re good though.

No, do not just replace the cat but not the sensors. Find a mechanic with a graphing scanner who can tell you which is bad.

My 2002 Sienna had a number of P0420 failures, and in every case, for 220,000 miles the sensors fixed it.

If a mechanic tells you P0420 ALWAYS means a bad cat, run, do not walk, to the nearest exit and look for another mechanic.

Multiple sensor failures are abnormal, perhaps the person doing your diagnostic work is no better than the person who told you “P0420 ALWAYS means a bad cat”. From past discussions, I recall that your vehicle’s ECU (PCM) has a software flaw that causes the P0420 fault to occur.

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Major Internet outage for 3 days. Yes, that computer had a wrong spec programmed into it. I asked about it, because supposedly it was able to be re-flashed, but was told on that particular one it could not be flashed, so I would have to buy a new computer. I did not.

I did the troubleshooting myself. Every time I got P0420 I did not run and replace parts, but waited to see what happened.

After one long run down hill going into Vera Cruz, I started getting the P0420 off and on. It would fail and after driving it a while, would be okay again, then it would fail again, over and over.

Finally one day on I-80 when I started the car after a stop at a gas station, the heater in one failed. Since it was drivable, I drove back to McAllen and had it replaced there. No more intermittent P0420.

Note I said I had a number of ‘P0420 failures’, which you changed to ‘sensor failures’.

I previously had other P0420 failures, my sensors started failing around 100,000 miles, and I drove that car 220,000 miles. After the first sensor failure, I forget, but I think around 100,000, I went ahead and replaced the other one as PM.

I guess I was wrong to presume that you meant more than one sensor failure.