Hey just looking for some direction. Went to have my 02 Corolla inspected and it failed the emission check because of a P0420 code.

Car has 183k miles but is in overall excellent condition. Gets 25-30 miles a gallon city and 40 highway still. Was pretty surprised by this failure as there is no check engine light, went to autozone and their cheapo scanner said the same thing and to just replace the catalytic converter.

When I bought it at 134k miles the check engine light came on, on the drive home. Took it back and they cleaned the MAF sensor and it was good to go. This was about 4-5 years ago.

Since it's last inspection it's gotten an after market gas cap, as the original somehow disappeared.

Is there a way to check the sensors to see if they're bad?


I've been needing a code reader to clear codes anyways. I also have a 2004 Mach 1 with an abs light on, I had the Harbor freight abs code reader but it wouldn't read Ford codes, are there any I can buy that will? Preferably do both cars.

replace the O2 sensors.

P0420 is the cat not responding as fast as it should to control NOX. Replace the cat, in Texas you can probably get by with a good aftermarket cat. In states like California you are pretty much stuck with OEM cats to pass the smog inspections.

Replace the O2 sensor

Yeah, I wasn't sure since the code seemed vague and from Google I've seen mixed things.

Even with no check engine light? The 02 sensors are much cheaper that a cat.

Thanks

What's an emission check?

It's where they open your wallet to see how much money you have in it.

Then they take it.

F'ing stupid is what it is. I'm originally from FL where we don't have any.

The jets I fly burn thousands of pounds of fuel an hour with almost no exhaust or emission control, yet this little 4 cylinder car is a problem.

so... you fly jets for a living and you drive a 02 corolla.  why?

Get a cheap bluetooth OBD2 adapter, download the Torque app to your phone, and post the graphs off of your O2 sensors.  

The trace off of the front sensor should switch back and forth between about .1 and .8 volts.

The back sensor should be flat around 0.1 volts.  If the back sensor switches back and forth like the front one, the converter is bad.

ETA - since it's a Toyota, the front sensor may be an air/fuel sensor instead of an O2 sensor.  If so, you will see a flat voltage off of it around 3.3 volts.

ETA2 - for Fords, use the same bluetooth adapter and use Forscan.  It will access nearly all of the Ford modules, including ABS.

Great, thank you for the info. I've been doing a little more research and that all makes sense. I didn't have much of a chance to mess with it today but will see if I can check over some things tomorrow on it. I've been eyeing those Bluetooth sensors and will get one inbound from Amazon.

Air filter is about a year and a half old, battery is about the same. Will check the pcv valve and clean the MAF sensor while I'm at it, and terminal connections. Pretty sure I changed the spark plugs when I put a new filter in it but not 100% sure.



This is my cheap airport car I can leave in the airport parking lot and not worry about door dings or paint fading. I have an '01 Bullitt and '04 Mach 1 I drive when I feel like it.

P0420 is almost always a toasted cat.

It sets when the downstream O2 shows a similar waveform as the upstream O2.

Good cat: Upstream O2: /\/\/\/\/\/\/\/\/\ Downstream O2: ~~~~~~~~~~

Bad cat: Upstream O2: /\/\/\/\/\/\/\/\/\ Downstream O2: /\/\/\/\/\/\/\/\/\/\

Thanks for all the help pointing me in the right direction. Got a obd 2 sensor on the way and will check the 02 sensors this weekend when it arrives.

P420 and P430 almost always cat/s.

Toyota name brand wont protect you from this. Shit breaks down/fails no matter what brand.

This will fix it

Straight or angled CEL fix is what you want. 

P0420 is the downstream O2 sensor reading too many crosscounts indicating an inefficient CAT. Above link will fix it. P0420 is the No. 1 bank which is the bank with the No. 1 spark plug. It's on YouTube. You don't have emission tests do you? 

If a cat looses efficiency by a few percent the EPA nazis mandate your car throws a code causing it to fail the smog.

Add a gallon of lacquer thinner to at least 10 gallons of gas on your next fill up.  Drive your car around a whole bunch of places making sure it gets fully warmed up. Once you get enough drive cycles in the code should clear on its own. Go back and reinspect.

This may clean your cat enough that it gets you through the inspection.
If you want more info on lacquer thinner and this method there are a ton of YouTube videos.

Or replace the cat, but it ain't gonna be cheap. Cats have precious metals (palladium?) and are not cheap.

PO420: Your car is stoned.

My old neighbor was an automotive tech and he swore by laquer thinner/acetone.

+1 on this.  move your O2 sensor out of the stream.

We do have emissions test in Texas, or at least in my county (I think things recently changed).



Update:

Car passed inspection today! I ended up taking the advice from this thread, ordered an obdII wifi reader from Amazon for $17 and downloaded an app for $10. Got live data showing both O2 sensors were receiving proper voltage, had good MAF airflow. I bought some MAF/Intake cleaner but never got around to cleaning them.

Anyways, got the reader from Amazon last Friday and cleared the P0420 code. I then drove about 200 miles over the weekend in it. No Check engine light code or return of the P0420 code. There were no CEL codes in the app or the light originally, just the P0420.

Took it into Midas today and they checked for exhaust leaks and hit the cat with a laser thermometer. The tech said it was the right temp before and after the car, and didn't recommend replacing it. The car has run all the self tests except the catalyst test hadn't completed, but they said it can pass with only one test not complete or passing. Since this was the only one, I should be good. He said he'd run the inspection there and it should pass but he noticed the CEL light didn't come on when the vehicle was powered up and he can't pass it strictly off the dead light bulb in the CEL. BUT, if the shop that initiated the inspection didn't notice it, they may pass it with it still being dead.

So I drove home, got my obdII reader and hooked it up. It had completed the catalyst test and passed, I selected TX in the app and it told me I was ready to pass inspection. Immediately drove to the shop and it passed inspection.

So not really sure what caused it to come on, I hadn't been driving it much before going to the inspection. But it's stayed off and runs just fine.

Then I got some light bulbs to fix the CEL, here's where it gets interesting. When I bought the car from a used car dealership about four years ago, I had it dropped off after getting "new" used tires as the ones on it were bald. It test drove fine and ran well but when they dropped it off the CEL light almost immediately came on. No warranty, but I called the shop and gave them the wtf speech and they agreed to fix it anyways. Took it in and they "cleaned the MAF sensor etc" and the CEL light stayed off and I was happy, car drove fine and no more CEL light.

I did some work to it a year and a half ago, steering rack, valve cover gasket, plugs, filters etc. Was in FL at the time and there's no inspection there. My mechanic said it was good to go and I drove it back and forth across the country pulling a trailer with no problems other than burning a little oil.

So I took the gauge cluster out today to replace the bulb, there was no fucking bulb in the CEL location. It was a plastic clip that rotates the bulb into place on copper contacts. I could see where the contacts had a bulb installed previously and there were some that were unused that didn't have the rub marks of the black plastic clip rotating into place. So the dealership F'ing pulled the bulb out in case the car had further issues, and I just wouldn't know.

Ended up pulling the bulb from the ABS light and putting it into the CEL light location until I can get a new one. So it's working properly now, I disconnected the battery to pull the gauge cluster so now the car will be running the self tests all over again. I'll keep an eye on it but so far it seems to be fine.

Thanks for the help everyone. Now I'm gonna use this obdII sensor and app to fix the ABS light in my Mach 1. Already got a code for it, now going to dig a little deeper.