Stock exhaust P0420 code. Help!

2001 mustang GT stock exhaust with cats. Engine light was on when I bought it. P0420. My brother reset it. 47miles down the road, CEL light came on. My brother and his scanner is out of state so I went to autozone, once again P0420 code. Disconnected the battery numerous times. CEL light kept returning. First time at 47mile then 37, 25… I bought a pair of mil eliminators from American muscle, because I need the car to pass inspection. Disconnected the battery for 25min while I installed them. Hooked the Battery back up and drove about 30miles and the light came on. Went to autozone for a code read, P0420… Any help would be appreciated… This car is becoming a major headache. I don’t have the money to bring it to a shop or to buy 4 new o2 sensors.

And when you disconnected the battery, it means all the readiness monitors have to be reset. Otherwise the vehicle will fail the emissions test.


Yup, it’s about the right age for new cat converters…

It’s going to continue failing until you actually change some parts. If you’re lucky, you’ll only need oxygen sensors. But don’t be disappointed if you discover you need new cat converters. You might want to consider shopping for them online and having a knowledgable buddy help you change them. My guess is that the guy that sold you the car got a quote from the dealer, and, after he was released from cardiac rehab (the price probably gave him a heart attack) decided to sell the car instead. Fortunately for him, you came along…

Consider this a tuition bill in the art of buying a used car… never buy a used car with a check engine light on, and never until it’s been thoroughly checked out by a trusted mechanic. A pretty car bought without following this advice can quickly become a pretty expensive car.

“I don’t have the money to bring it to a shop or to buy 4 new o2 sensors.”

This is the biggest issue.
If you don’t have the knowhow to deal with a problem you have to rely on others with more expertise.
No way around it. Car ownership is a substantial ongoing expense.
Blindly changing parts (sensors) can be a costly crap shoot.
Help over the internet has its limits and sometimes you gotta pay the piper.

You need to find someone who knows how to read the signals from the O2 sensors and figure out what’s really happening.
P0420 code could be caused by something as simple as an exhaust leak.

By “mil eliminators” you mean the extender for the downstream O2 sensor?
The fact that this didn’t stop the light is evidence that the problem is not a cat converter.

I understood that to mean the straight pipes that replace the cat converters. One can expect the CEL to remain illuminated with these pipes, as the oxygen sensors will not detect a change in oxygen levels between the first and second sensors. That would cause an illumination even of one was not already present.

A P0420 is a generic code for “Catalyst efficiency below threshold” this is coming from memory so I’ve seen the code a “few” times. What it signifies is that one of the o2 sensors is either dead or faulty…OR your cats are bad…OR there is an exhaust leak somewhere.

The cats being bad is really not common…I very rarely see cats go bad tho Im sure others will argue with me on this. Unless the cats have been tampered with or hollowed out or had accident damage…they usually last the life of the vehicle…quite literally.

I would start at the O2 sensors. However without the proper equipment…Read expensive computer usually at the dealership…its hard to pinpoint which o2 is the culprit. Basically what this code is saying is that the o2 sensors are not noticing a reduction in Nox from the pre cat O2 and the post cat o2… Usually I find that it is the post cat O2 as the issue…thats just me… it can go either way actually.


“I bought a pair of mil eliminators”

in my opinion, this sounds like some kind of plug in device, or something that’s spliced in, which is supposed to keep the Malfunction Indicator Light from illuminating. Perhaps it’s something as devious, as a device which modifies the signal from the downstream sensors, so that the PCM sees them returning a nice steady signal, instead of a wildly fluctuating signal. And that wildly fluctuating signal might be because of a bad cat

It may be also be a device which supposedly keeps it from ever turning on

If that is the case, OP will certainly fail the bulb check. Which means he will fail the smog inspection at that point. Just because the mil isn’t lit, doesn’t mean there are no codes. The presence of P0420 will also cause the car to fail smog inspection

Bottom line . . . pay a shop to diagnose and repair the car properly

One more thing . . .

When you do bring the car to a shop, be upfront

Tell the service writer that you installed a set of “mil eliminators” . . . whatever those may be

If you don’t tell him about it, it may be to your detriment. The mechanic may take longer to diagnose the car, because he probably won’t be expecting tampering. The longer he takes to diagnose, the more you’ll be paying

Not to mention, if you don’t tell him, and he finds those devices later on, it may not be very pleasant. Because you knowingly withheld information, which would have made his job easier


The type of “mil eliminators” I’ve seen are modified spark plug anti-foulers.
They shroud the downstream O2 sensor out of the exhaust stream.
Another type adds a resistor-capacitor low-pass filter to the output line.
This keeps the sensor output from changing rapidly, which triggers P0420.
A google or youtube search will reveal what I’m talking about.

UH…Why dont you just repair the problem? Its not that difficult. It will SAVE you money in the long run. Your fuel injection needs this information so that it can properly calibrate itself and not just run super rich all the time and waste fuel.


Sorry it took so long to reply, i often have trouble using this site from my iphone. i log in but everytime i go into this area it says “Log in or sign up” on the bottom. Anyway…

@Tester it also wont pass inspection unless all but one moniter is ready. On older cars you can disconect the battery and pass inspection. Assuming the shop doesnt see any other problems.

@“the same mountainbike” I have stock cats on the car, if you look at circuitsmiths last post he explains what they are… If the cats are just less than 95% efficiant, that could set off the light. In many cases, people waste a ton of money replacing the cats and the light returns. The car has 97k miles on it and shows no signs of serious catalytic blockage, thats why i got the Mil Elliminators. A lot of people who have mustangs get rid of the cats and replace it with straight pipe or high flo cats. The purpose of the mil elliminators is it basically tells the computer everything is fine, so it wont set off the service engine/Check engine light. If the cats are indeed bad, the Mil Eliminator are supposed to keep the computer from knowing it. Also from what i hear, if the sensor was bad you would get a different code than P0420. But i dont know that for certain, so that is still a real possibility imo. Im currently looking into wether the mill eliminators require a fully fuctioning sensor in order to do its job. But it would seem to me if the sensor is setting off a code, the Mil eliminators should work.
Also, someone could easilly just disconect the battery terminal and hide an engine light. Everytime i reset it that way, it takes at least 20+ miles for it to come on. But yea i saw the light on when i bought it. Like i said, a lot of guys with mustangs cut the cats off and put on mil elliminators, something id like to do eventually. Most of us know inspection shops who dont even look under the car, they just plug in the OBD2 and do a basic safety inspection. At least i know a lot of places that do that… So the possibility of bad cats wasnt a deal breaker for me.

@circuitsmith Im not a mechanic but i manage by watching how to vids and help from people on forums like this. Im not an expert and dont have the funds to drop it off to have someone else figure it out and repair it for me, that means i shouldnt buy a car? lol. If i could afford to pay the piper, i would. Sue me for trying to figure out the problem before pissing the little money i have into the car. I thought this was a place for people like me. And you make an interesting point that the mils not working points to it not being a cat or sensor problem, but what else sets off a P0420 code? I know there is a range of possible problems, but i thought they all were triggered by the downstream o2 sensor i was trying to bypass with the mils…

@“Honda Blackbird” Im still considering the possibility the post o2 sensor is the problem. The mils may require that sensor to fully functional in order for the mil elim to work. I figured its setting off the code so its working enough, but i could be wrong. It could indeed be faulty. im gonna also look into having someone turn the sensors off with a tuner. As for your second comment, the reason im here is because i dont know what the problem is, lol… A lot of great advice and things to look into. But its not a sure thing that its an o2 sensor at this time. Its a possibility.

@db4690 Circut explained how the mil works. I got the plug and play ones for $50. Some use sparkplug anti-foulers. I regret not doing that, because its cheaper ($10). Paying a shop is not an option. I would rather slightly overpay for an inspection sticker than spend hundreds i dont have at a shop. The light is annoying but the car runs great. Id like to get the light to go out and get it to pass normally if its not too much work or money.

I apreciate all the advice from ya’ll :smile:

These are the mils i bought btw >

Interesting. You insert them between the rear O2 sensor and the harness then? It might work for keeping the CEL off, but not sure about for DMV required emissions testing, as the car’s computer may flag it as a missing signal when the emissions computer asks for the emissions related OBD II data. I doubt this would work in California. But it might work in other states. Worth a shot anyway.

My bet is your cats are ok but there’s some kind of mixture or exhaust problem. Do you have access to a scan tool that will give you the measurement of the extra fuel needed – beyond what the computer thinks it should for that condition – to make the O2 sensor reading ok? “Fuel trim” I think is what that parameter is called. If so, post it here, that might provide a clue. Assuming all the engine maintenance work is up to date, no signs of misfires or missing or pinging esp on acceleration or going uphill, the other thing I’d focus on is exhaust leaks. Especially any leaks before the first O2 sensor. A tiny crack in the exhaust manifold could cause this, or a compromised exhaust manifold to block gasket.

Do you have any clues that indicate only one side of the exhaust is causing the problem?

@GeorgeSanJose Yes i hooked them to the rear o2 sensors. Its supposed to trick the computer into thinking everythings ok. Like it gives a steady signal or something like that… I think the cats are ok too, the car runs very well, nothing seems blocked in the cats. no misfires, missing or pinging that ive noticed… I dont have access to any scan tools atm. My brother who will be back in ny in a few days has one of them scanners that checks the engine light, not sure what else it can do… if you mean like a tuner, i dont have access to anything like that. I heard the o2’s can be fixed or shut off with a tuner, im gonna look into that. I dont hear any leaks, but I will have to jack the car up and look over the exhaust while its running. But even if there is a leak, the Mil elliminators should take care of that. Theres a possibility i got a bad set of mils… I switch them from side to side and see if that works. would be odd getting two bad ones. The only clue is the P0420 code says “Bank 1” I hear thats the passenger side.

Bank 1, well that narrow it down a bit. Check carefully for signs of exhaust leaks on that side.

One idea, you could switch the O2 sensors from bank 1 and bank 2. If the code then reads bank 2, you know it is probably one of the o2 sensors that has failed.

I’ve never had to check for exhaust leaks myself, so I don’t know any tricks for checking for exhaust leaks that would apply to a diy’er. I think shops might use an emissions sniffer. Maybe somebody here has done this before and has some tricks.

You do realize this is illegal?

A DIY method to check for exhaust leaks is, start the engine, take a hunk of 2X4 and block the tail pipe. If there’s an exhaust leak, it’ll show up pretty quickly.


A gloved hand over the tailpipe can do it too.

I don’t mean to sound like a jerk . . .

The way to get the car to pass smog inspection normally, is to remove those “mil eliminator” devices, diagnose and repair the problem

What you are looking to do is cheat the system and save some money

You can rosey it up and use fancy words, but that’s exactly what your aim is

As far as your idea that there are lots of Mustang owners that do what you do . . . I think it has less to do with the particular car, versus people aren’t willing to pass smog legitimately


If the cats are just less than 95% efficiant, that could set off the light.

Where on earth did you get that piece of misinformation?

Like i said, a lot of guys with mustangs cut the cats off and put on mil elliminators, something id like to do eventually. Most of us know inspection shops who dont even look under the car, they just plug in the OBD2 and do a basic safety inspection.

The fact that lots of people do something illegal and the fact that there are incompetent and crooked shops out there isn’t going to get any sympathy here, I believe.