Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Po420 code came again! New plugs, coils, and EGR cleaned HELP!

Hi there,

About a month ago, I had the PO420 code and put in a new ignition coil, the code went away for like three weeks, I was so happy I thought it was fixed.

Then yesterday, after a long drive, it mysteriously reappeared.

My mechanic said the plugs are ok, I have new coil in there, and the EGR valve was cleaned out.

Prior to cleaning the EGR valve and replacing the ignition coil, its acceleration was misfiring under a load , but after the egr valve cleaning and new coil installation; it accelerates smoothly under all circumstances.

What should I check now? CAT? TPS?

The car has to pass emission in October, I may buy one of those 02 sensors that erase this code.

Its a 2000 protege 1.6L engine, car has 221 200 kilometres .

I dont like throwing parts at a car!


A DTC P0420 indicates that the catalytic converter efficiency is blow threshold.

So with a vehicle with that many miles, what makes you think that the catalytic converter itself isn’t causing the code?


How long do cats last for?

I read in a post earlier that cats are very durable.

If thats the case I will buy one of those 02 sensors that change the resistance and remove the 420 code…

cat is the last thign I want to replace.

The cat that came in the vehicle was warranted for 8 years/80,000 miles.

So they don’t last forever.

Why don’t you fix the vehicle correctly with an aftermarket cat?

Instead of being a hack?


I agree with tester on this one.
You need a catalytic converter. Might’s well bite the bullet and go for it.

It’s good to hear you’ve solved the operating problem. Time to solve the emissions system problem. :slight_smile:

A catalytic converter can last the life of the car. I’ve had them last well over 300,000 miles. However, if you operate a car with a misfire you can wipe them out in very short order.

221 kilometers, what’s that, about 150,000 miles? That’s sort of on the border for when a cat might fail, but there’s some chance still this isn’t a cat replacement problem. Before replacing the cat, suggest as an alternative to first install new spark plugs, new engine air filter, and ask your shop to clean the throttle body and air flow meter. If this engine uses a distributor, add in a new distributor cap and ignition rotor. And all new plug wires. And I install a new EGR valve too, since cleaning the old one seemed to help. Those EGR valves live in such a harsh environment sometimes they can’t be effectively cleaned and expect the cleaning to take. I’d probably replace the thermostat too, if it were my car with this problem.

Anything’s possible, but unlikely to be the TPS unless there’s a TPS code present. You might ask your shop to check the coolant temp sensor and the intake air temp sensor. Those two sensors rarely fail though.

Worse case you replace all that stuff and the code remains. Replace the cat, and you have a car with most all the normally wearing parts replaced and a new cat. And doing all that other stuff won’t break the bank either. It’s routine maintenance stuff. You should be good to go.

Well what are the tell tale signs that it is the cat? aside from a
420 code which can mean anything…

George—>> mechanic said plugs are good. and air filter is new.

Interestingly, it was after an hour and a half long drive (highway
driving) that the code reappeared.

GEORGE>>>> Thermostat is new too

It’s not accurate to say that anything can cause a 420 code. 0420 means the ecm has noticed a problem when comparing between the o2 sensor reading upstream from the cat vs the o2 sensor downstream. When the cat is working correctly, there’s supposed to be a certain known relationship, which apparently isn’t the case for this vehicle. There’s other things than a failing cat that can cause this discrepancy in o2 sensor readings, upstream/downsteam however. In addition to the o2 sensors themselves – which are not a common failure item – anything that prevents clean and complete combustion of the air fuel mixture can fool the ecm into thinking there’s a cat problem.

fyi, shop’s have test equipment to futher narrow down the cause of an 0420. One test they’d usually use is called a fuel trim test.

Given your history of misfires the catalytic converter may be damaged. The performance of the catalytic converter can be monitored with a scan tool, you just need a technician with training to check it.

Oh and yes I did replace the upstream 02 sensor … one of many parts
I replaced.

code 420 shows that the resistance reading from the 02 sensors are not
within spec correct? (no Im not a licensed mechanic)


re: resistance reading from O2 sensor

The most common type of O2 sensor outputs a voltage related to O2 concentration in the exhaust gas, not a resistance. An O2 sensor before the cat and one after allows the ecm to tell if the cat is working or not, by comparing the two outputs. The one after the cat – providing the cat is working – outputs a different signal than the one before. If they output the same, that would mean the cat wasn’t doing anything and would post a 0420 code. I think what you are referring to is a presumably illegal gadget that causes the post cat O2 sensor to produce a signal that looks like the cat is working, whether it is or not. And that device may involve using a series resistor to produce the desired affect. I’m not sure as I’ve never seen one. Neither of my vehicles is OBD II, and so neither uses a post-cat sensor.

The other place you might have seen resistance associated with an O2 sensor is the sensor’s heating circuit. My Corolla’s O2 sensor is unheated, so it has just two wires connected to it, one for the output signal, and one for ground. O2 sensors are not accurate until they get hot. That type of sensor uses the engine heat to heat it up. Many O2 sensors these days are heated electrically to make them work faster after starting the engine, and that is done with a resistance heater. So there’s an extra wire or two going to that type of sensor, for the heater circuit. The ecm measures that resistance and if it is out of spec is will post a code for the O2 sensor heater circuit. That isn’t related to the cat monitoring function, except that it can’t monitor the cat if the sensor heating circuit isn’t working.

OK something you guys should actually know:

It is burning oil---->> when I start the car for the first time in the morning (cold), fumes of white smoke come out for like two minutes…so that would suggest internal leaking of oil?

Yes and I know burning oil has the ability to damage the cat?

Its tough because I live in a small apartment building, where I have to park outdoors: I dont have a nice garage to work in.

So I think it is wise to test the cat…

What is the easiest way for an individual without jacks to test the cat?

temperature test?

and fuel efficiency appears to have deteriorated

It’s normal for some water vapor – steam – to come out the tailpipe when first starting the car & it is cold outside. And oil burn smoke isn’t white, it is brownish to blue-ish. Are you having to add more than one quart of oil per 1000 miles? If not, your engine isn’t burning oil.

There are no easy ways for a diy’er to test a cat and get a categorical yeah or nay. It’s usually better to let a shop do it for you. Either that or just replace the cat if you think yours is shot. If the replacement doesn’t fix the problem, worst case you’ll have a spare cat for when the current one fails.

Shop’s test a cat with sophisticated scan tools, but diy’ers don’t usually have access to a scan tool that’s good enough to test a cat. Shop’s also have tailpipe emissions testers, which is another good way to assess the cat performance. A diy’er could get some sort of assessment I suppose by measuring the cat temperature after a metered length freeway drive at 65 mph, assuming you have a known working cat from the same car/engine to compare against. The cat back-pressure is a figure of merit for cat function. Do you have the proper equipment to do that test? The intake manifold vacuum reflects the cat back pressure and so is related to cat function. It’s not specific to the cat is the only problem.

If I had a cat performance issue and I didn’t have any other way to test it, I’d use a lab o-scope and compare the waveforms from the pre-cat O2 sensor to the post-cat O2 sensor. As as baseline I’d do the same experiment on a engine – preferably the same engine design – configured w/ a known good cat.