P0421: Warm Up Catalyst Efficiency Below Threshold (Bank 1)
- Thread starter gova
- Start date
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- San Antonio, Texas
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- '15 CX-5 Miata AWD
manual says
POSSIBLE CAUSE
l Exhaust system leakage
l A/F sensor loose
l HO2S loose
l TWC deterioration or malfunction
l PCM malfunction
check/clean maf.. if your cat is bad it should be under warranty
it also lists some other details but they are more specialized equipment items..
POSSIBLE CAUSE
l Exhaust system leakage
l A/F sensor loose
l HO2S loose
l TWC deterioration or malfunction
l PCM malfunction
check/clean maf.. if your cat is bad it should be under warranty
it also lists some other details but they are more specialized equipment items..
Late to the party, but, before you panic, I had the same thing pop up with some regularity on my Speed 3 - in fact, I could almost bring it up at will. Just putt around slowly, say, going through a residential area @15mph after stopping for a few minutes at a convenience store/yard sale. It wouldn't bring the cat up to "temp" rapidly enough and would upset the ECM = PO421. YMMV
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- 2014 CX-5 GT AWD
P0421 diagnostic help please: On my 2014 CX5 2.5 at 91,000 miles, I've also been getting P0421 consistently (I have a little gauge that monitors). No other codes come up to help diagnose the issue. It shows the code as pending on each drive and as a CEL. I started by changing new factory spark plugs, air filter, and cleaning MAFS and resetting. I haven't changed any O2 sensors yet. Do I change $360 in sensors just to see if that fixes it? Is there a better way to diagnose the issue without spending a huge pile of cash?
It seems pretty early for a cat to go bad. Could that really be what the dealer will tell me, and will the code come back from whatever possibly caused poor "warm up cat efficiency"?
Thanks for your help.
-Broke teacher.
It seems pretty early for a cat to go bad. Could that really be what the dealer will tell me, and will the code come back from whatever possibly caused poor "warm up cat efficiency"?
Thanks for your help.
-Broke teacher.
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- San Antonio, Texas
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- '15 CX-5 Miata AWD
I would put a strong cleaner such as G2P (guaranteed to pass) which can clean into the exhaust manifold before dropping $ on new parts. I use it every few mo to keep my cat/02 clean
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- 2014 CX-5 GT AWD
Do I assume correctly that you put the G2P in the fuel?
Thanks
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- 2014 CX-5 GT AWD
Upon further reflection, could this also be related to a carbon build-up issue? I bought the car used and it showed carbon in the exhaust tips. I thought "what's up with that?" I think this problem surfaced shortly after I tried spray cleaning carbon through the system (unaware of an issue with SkyActive or GDI), rather than removing the intake manifold. I removed the hose after the MAFS and used CRC - for FI and Throttle bodies but not the kind designed for GDI, again because I was unaware that there was a carbon build-up issue.
I did this with a helper and the engine running at 2-2500 rpm. Perhaps too much unburned carbon and fuel went downstream. I thought it would burn up over time and hoped the cat would be OK. It has not; it's been an oil change and another half - maybe 5 months of a mix of stop and go, hard acceleration, and high-speed highways. The code still comes up consistently on each drive.
Do you think I need to do a manual intake valve carbon cleaning like the one on the below video before the G2P?
I did this with a helper and the engine running at 2-2500 rpm. Perhaps too much unburned carbon and fuel went downstream. I thought it would burn up over time and hoped the cat would be OK. It has not; it's been an oil change and another half - maybe 5 months of a mix of stop and go, hard acceleration, and high-speed highways. The code still comes up consistently on each drive.
Do you think I need to do a manual intake valve carbon cleaning like the one on the below video before the G2P?
Are you just trying to pass an emissions test - or trying to understand what repair is needed to remove the P0421 code?
Either way the guaranteed to pass stuff is worth a try - if only because it is a cheap try and as it is burned with your fuel it does travel through the cat.
Don't mess with the intake valve cleaning unless you are getting misfire codes and really poor running engine. If the valves have excess carbon they will either not seal or reduce intake air to the point of misfire. Carbon build up on the valves will not impact the catalytic converter or cause the P0421 code.
The P0421 code is a catalytic converter is failing code. It is one of only a few diagnostic codes that points directly to the source of the problem. It has nothing to do with spark plugs, MAF, etc. A tune up can help reduce tailpipe emissions, but won't fix this code. Don't read too much into the "warm-up" portion of the code. The real message is the catalytic converter is not doing its job of cleaning your exhaust of unburned hydrocarbons (think "fuel"). How does the ECM know? It monitors the oxygen sensor before and after the catalytic converter. If the downstream sensor is a mimic of the front sensor the ECM concludes that the catalytic converter is not doing its job. Since the ECM depends on the O2 sensors some people figure new O2 sensors might solve the problem. But, you have no O2 sensor codes (or any other codes) so changing the sensors is a long shot. Why did the catalytic converter fail at less than 100K? That is hard to say, but pouring raw fuel into the exhaust can ruin the catalyst. This can happen when an injector is stuck open, or the engine is run for a long time with a hard misfire due to failed ignition coil.
Either way the guaranteed to pass stuff is worth a try - if only because it is a cheap try and as it is burned with your fuel it does travel through the cat.
Don't mess with the intake valve cleaning unless you are getting misfire codes and really poor running engine. If the valves have excess carbon they will either not seal or reduce intake air to the point of misfire. Carbon build up on the valves will not impact the catalytic converter or cause the P0421 code.
The P0421 code is a catalytic converter is failing code. It is one of only a few diagnostic codes that points directly to the source of the problem. It has nothing to do with spark plugs, MAF, etc. A tune up can help reduce tailpipe emissions, but won't fix this code. Don't read too much into the "warm-up" portion of the code. The real message is the catalytic converter is not doing its job of cleaning your exhaust of unburned hydrocarbons (think "fuel"). How does the ECM know? It monitors the oxygen sensor before and after the catalytic converter. If the downstream sensor is a mimic of the front sensor the ECM concludes that the catalytic converter is not doing its job. Since the ECM depends on the O2 sensors some people figure new O2 sensors might solve the problem. But, you have no O2 sensor codes (or any other codes) so changing the sensors is a long shot. Why did the catalytic converter fail at less than 100K? That is hard to say, but pouring raw fuel into the exhaust can ruin the catalyst. This can happen when an injector is stuck open, or the engine is run for a long time with a hard misfire due to failed ignition coil.
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- 2014 CX-5 GT AWD
Hi Davmac and others,
No, I'm not currently due for an emissions test until later in the year, but when I take the car to the dealer they are sure to note the issue. The code came up a bit after trying to spray clean the carbon downstream through the system with CRC EFI cleaner (not the right GDI type cleaner) after a reset, so after finding out more about this issue I thought I fouled it that way. I'm not getting misfire codes and it runs very strong during all types of driving, only it seems a bit lean or not perfectly smooth at idle but not at all missing; it seems to be running perfectly normally. New spark plugs seemed to make no difference. The only difference seems when I reset the code during the "learning" phase, the idle seems only a bit smoother. It's always had carbon build-up on the exhaust tips. That's why I thought it needed cleaning, and what led me to believe it might possibly still be polluting the catalyst. No?
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Quick reply: Even if the intake valves have some carbon build up this is unrelated to the catalytic converter. If you are prioritizing repairs: save up for a new catalytic converter and don't worry about the black inside of the tailpipe or possible carbon build up on your intake valves. Neither of these is causing any issues to performance, economy or ability to pass emissions.
Now, if you have the tools and knowledge to clean the valves yourself then go ahead - no harm even if it is unnecessary. Just know that the P0421 is NOT going to go away.
Now, if you have the tools and knowledge to clean the valves yourself then go ahead - no harm even if it is unnecessary. Just know that the P0421 is NOT going to go away.
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- 2014 & 2019 CX-5 Touring(s)
Absolutely. Cat converters ARE covered under the emissions warranty.
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- San Antonio, Texas
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- '15 CX-5 Miata AWD
Get ready to pony up if you have to replace the em; its extra effort to remove from awd cars. G2P is a strong cleaner, the name is just self marketing for vehicles with poorly/never maintained fuel systems
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- 2020 CX-5 GT AWD
Had the same fault recently. Cleared it two-three time but it came back on. The engine felt lazy, lack of power, slow acceleration. Possible fix, the diagnostic said, reprogram the PCM. I did not want to give in and spend the money for that so I kept looking for some other cheaper ways to fix the problem or make it go away. The idea being, start with the cheapest thing and go progressively. And then it dawned on me: 02 sensor, the car thinks that it doesn't get enough oxygen, right? How about the engine air filter! I live in a metro area where we just had the pollen season and I was thinking that maybe I could/should replace it. Replaced the air filter and the car seems happy again, engine power is back, acceleration normal. Drove it for two days now, like I stole it, with no problems.
I hear ya about the pollen and whatever else is in the air. I change my air filter every 20,000 miles and notice a slight improvement every time, same as in the past with previous cars. Nothing scientific but it seems to be a correlation with engine displacement. The CX-5 and the GM 3800 V6 filters are about the same size. The Mazda 2.5L is 2/3 the displacement of the 3800 (3.8L) and I change the filter roughly 2/3 as often (20K vs 12-14 K on the 3800). Then again, maybe I've just invented another automotive myth.
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- 2020 CX-5 GT AWD
Well, I was hoping that the air filter would fix my problem. It did not. Few days later the P0421 code came back. Tried some catacleaner, it worked. For a day. Next morning, the fault came back. Today I stopped by my local dealer for a "professional diagnostic", hoping that they would tell me something that I don't already know. They did not. I've asked for a PCM reprogramming and they said 190$ diagnostic and the money will go against the PCM reprogramming price which was 190 so 190 total for a diagnostic and a reprogramming. Good deal, i said. I've also asked if I could go and talk to the mechanic myself, hoping that I can see the actual diagnostic or reprogramming and maybe have some questions for the mechanic etc. I was denied. Whatever they did to my car, 40 minutes later my car is out of the shop, I'm going in to pay. The little guy shows me a piece of paper with P0421 code on it and recommends changing the catalytic converter ( of course!). How about the reprogramming? Oh, yeah, we did that, too! Good deal! How much? Nothing, he said. If the light comes back on, you come back and then we'll talk about replacing the catalytic converter... As you probably already guessed, few miles from the dealer, the engine light comes back on, same code. So I'm thinking out loud: these mofos did not do anything, no reprogramming or diagnostic, they just cleared the code and off you go! And that's why I wasn't charged anything! Am I right? One of these days I'm going to crawl under the vehicle and inspect the downstream sensor and take some temperature readings from the converter. I still refuse to accept that the catalytic converter might be bad. This car is only 3 years and a half old.
Could it be the oxygen sensors going bad? That would be something you could do at home...
