P0420 06 Corolla Write up

[ryfly]

So this is a write up to how my corolla P0420 Code "Catalyst below threshold" was fixed for future reference.

Per online, and many other write ups, this code in general means you need a new Catalytic Converter.

So, engine symptoms of a failing cat is weak to bouncing idle, lower fuel economy by 5 to 8 mpg of your normal economy. (ex. going from 38 mpg to all of sudden 30), and During 2/3 to 3/4 throttle on acceleration-slight hesitation or surging between 3500rpm to 5000 rpms.

Main symptoms are the hesitation and bouncing idle.

This code is a two trip code but with a scanner will normally come up after 10 to 15 mile drive as pending during a diagnostic run after clearing, so this issue does not take to much time to set the light

Two trip is a cold, hot, cold, hot before is sets the P0420 code.


Now before i continue ALL CARS ARE DIFFERENT and do have a unique way of dealing with a bad cat or sensors.



Now lets talk of about what is happening when this code sets. During a driving cycle, your car runs it's own self emissions diagnostic or I/M readiness.

It tests many things such as Misfires, Fuel pressure, Cat converter, O2 sensor, and EVAP system and sometimes more.

Misfires and fuel system checks and normally determined within seconds. The Cat converter test takes about a 10 to 15 mile drive at 50mph cruise or more. After that test, it test the O2 sensor which takes another 10 miles after the cat test. After that its the EVAP which can take up to 50 to 100 miles and multiple drive cycles to test before a green emission light. If any of the test fail, a code is thrown immediately.

Now your ECU uses the MAF sensor (On the air box) to determine how much air, temperature and altitude your at to adjust fuel/air. Then your A/F sensor (Air/Fuel Ratio) located in the engine exhaust manifold uses the gases to work with the MAF to fine tune the air to fuel ratio of your engine for best performance and best cat converter cleaning. The a/f sensor is before the Cat converter. Your engine exhaust then goes through the cat and then checked by the O2 sensor. This sensor is checking the how well the Cat is cleaning the gases out. It compares the oxygen amount of a/f sensor, to the amount it reads.

Your a/f sensor in voltage will be switching fast and changing between 3 to 5 volts. Your O2 sensor should be VERY steady and be switching slowly at 0.1 to 0.8 volts but mainly should be steady. It should not be switching at any where near the speed of the A/F sensor. (ex. At idle, it should stay at 0.8 for 20 to 30 seconds then switch to lean 0.1 or .2 for 20 to 30 seconds while the a/f changes every second)

So if the O2 sensor is switching every 2 to 3 seconds if not faster, your Cat is not holding oxygen, thus not allowing the chemical reactions needed to take place.

Its also NOT UNSUAL to STILL PASS SMOG TESTS when a P0420 code is thrown. There is a second Cat converter which is not checked by anything and more or less is a back up after the Main cat. But it still helps to compare NOx readings from previous test when you had no P0420 code. (ex. Before P0420 i has NOx at 0.117 and then when P0420 happened, i still passed but my NOx had gone up to 0.3 with 1.0 being the fail point.) So yes, your toyota is being sensitive to how well it cleans the air, even if it is off a little bit.



Next is diagnostic. If you like to assume all sensors are working correctly, then replace the Cat and O2 sensor.

But YOU MUST DIAGNOSE why the Cat failed in the first place! Google it. Misfires, weak ignition, etc can cause unburned fuel to be dumped into the Cat and poison it. The cat is not designed to handle this. Cats normally last 100K to 200K and i've seen them last up to 300 to 400K miles.

First check the MAF by running your engine and unplugging it. Your engine should shut off immediately and run in limp mode. A OBDII live data scanner will help to.

Then disconnect the A/F sensor. Your engine should shut off immediately and run in limp mode. Check voltage readings with OBDII scanner if possible.

Also, check throttle body for dirt, and spark plugs, coil packs, etc to find out if they are operating correctly and not fouling and throwing unburned fuel into the Cat.

Next is the O2 Sensor. On Corolla's YOU DO NOT HAVE TO REMOVE THE SEAT. All you must do, is pull the carpet out a little from the center console, near the foot well. You will see where the O2 sensor wires go out and connect right by the air bag controller. With engine running, disconnect the O2 Sensor. IF the engine hesitation and idle problems go away immediately, vice versa if you plug it back in the problems come back. The O2 sensor is used during idle and mid range power to fine tune your engine performance just slightly but enough to cause slight engine problems. A Clogged cat would make no difference and engine problems would still persist whether connected or not but a poisoned cat, not clogged would cause the O2 and fuel trims/computer to try to correct this situation of a inefficient cat. This test along with voltage test, the best test to see how fast it is switching, will help in determining if the O2 sensor is lazy or weak to the Cat is bad. Normally, a weak O2 sensor would throw it's own code and needs the the right conditions for it not to throw it and instead throw a P0420 code. This is why you replace the O2 sensor with Cat sense either could be bad.

I first start with O2, put in a new one, Clear the code, and drive 30 miles to see if it throws a pending P0420 code. If so, then replace the Cat.

Sorry but yes, your Cat is bad and if a brand new, stock Denso O2 sensor does the same code, your Cat is bad. But I REPEAT you must figure out why it failed first or you risk ruining your replacement cat. On 06 Corolla's OEM cats cost about $800 which is a 5 foot section of pipe, two cats, and resonator. MY recommendation is OEM sense after market cats, usually cheaper, use less precious metals, and normally last only 2 to 3 years if not shorter. But if money is a issue, have a Muffler shop weld on or replace with a after market Cat. A OEM cat, if your mechanically inclined, not to fat, and can fit under a car on jack stands, you can replace it your self and save yourself 200 bucks in labor. Its only held on by 2 rubber mounts and 4 bolts.

BUT BE CAREFUL AND GENTLE if you do it your self. YOU CAN NOT BANG AROUND your new cat at risk you may hurt the converter. Also the bolts can be very rusted and tight!!!!! You run a risk, depending on the age of the car and the conditions it has been in, especially if near the sea, you can simply break and shear the bolts off which the requires you to replace the muffler or exhaust manifold since the actual hex bolts are welded on. So use penetrating oil, or bolt lubricate. Let it sit for 30 min, then gently with the best fit wrench, loosen the bolts.

You can normally replace the muffler gasket but the exhaust manifold gasket will normally be so tight and melted on, only way to get it off is to break it off, then using a dremel to sand and polish down so a new gasket can fit. Now if the gasket still looks in good condition, so no burn marks, scrapes, etc going on the outside of it, it is worth the try to reuse it. This is the gasket between the manifold and the Converter. The gasket between the pipe and the muffler can be replaced with out breaking it off. So buy a extra one just in case the reuse of the old one doesn't pan out.

A bad gasket will cause a exhaust leak and throw a P0420 code immediately along with noise and other symptoms. Reusing is a 50/50 chance you take. Its either gonna seal nicely or its not. If it doesn't, best to drive it to the nearest muffler shop for them to break if off and replace it. But as i said, best is to replace it, but no harm if to try and reuse it. Its your call.

After you confirm no leak and tight bolts. DO NOT OVER TIGHTEN. Usually a slight to moderate push is all thats needed since the heat expands against the bolts anyways and seals them on decently.

Clear the code, and take for a test drive When i say test drive, i mean a 60+ mile test drive and the most straight, consistent speed, highway, road you can fine. Use a OBDII scanner to check the I/M status. You will know within 60 miles if the Cat and O2 sensor pass without throwing a code. Also rough idling, and hesitation should be gone immediately.

If you have a tech stream or OBDII scanner with Live data, you can normally also monitor fuel trims, and sensor voltages. IF the cat was bad, you will notice a major difference in fuel trims as they will drop by 3 to 5 % less of what they where earlier. Short term will be more stable at around +/- 2%, best if stable at 0% at idle. Long term you want within +/- 10%. At higher altitude, its not uncommon for these to be within +/- 12%. Your in the goods if these became more stable and more in range.

After mainly under 3 drive cycles, normally, conditions have to be just right for the EVAP test, normally also after 70+ miles driven, with no code, you are in the goods.

Now your not out of the woods yet. Rule of thumb for me is after 5000 miles and still no code, as long as you fixed whatever in the engine first and still no code, you may breath a little easier since normally at 5K and still no fail, there should be no reason to go 100K to 200K with no cat failure and improved fuel economy. (ex. On the 06 Corolla, with 133000 miles, the fuel economy pre failure was 36mpg, but now 39 and if real good driving, maybe i'll hit 40. )



Also if your cat fails and you know it, before replacing, Do a Major throttle body clean, major fuel additive clean as well since your cat is already bad and your not gonna hurt it anymore. Have your mechanic or Toyota do this. You might as well drive a tank of gas on BGK44 and some other real nasty stuff to clean the guts of your engine out REAL WELL!!!!! Take a bad cat as a opportunity to really tune your engine back up to speed. I did. I did it all too. At 133K i had all first cleaned by some of the most intense throttle body cleaners which spray pressurized cleaner and put out TONS of white smoke. After that, rule of thumb anyways at 120K service, is transmission fluid, oil, brake, coolant etc flush. After you have cleaned the crap out of your engine and ran a full tank of BGK through, go ahead and replace the Spark Plugs, MAF sensor, A/F sensor and then the cat and O2. After all of this, your engine should be back to new condition and your car if it could smile, would smile.

Coil packs even at 100K do not need replacing for if not another 100K so all of the cleaning shouldn't effect them.

So if you have the money, and time, do this. Now all cars are unique to there problems to be sure to take time to diagnose the problems first before replacing.