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P0420 Code Again

By marathoner53, December 29, 2014



I have a 2003 Corolla with nearly 176,000 miles. Car has been great other than several instances of the P0420 code which ultimately led me to have catalytic converter changed twice. Last time was around 132,000 miles. No check engine light since until now and once again it is the same P0420 code. I haven't had full diagnostic work done yet, but I am afraid that I will be told to change the converter again. What other problems would cause this code? Both oxygen sensors were also replaced around 131,000 miles. I love this car and want to keep driving it as long as I can, but this is annoying. Thanks!

Low catalyst efficiency DTC can be caused by a number of things - bad catalytic converter, bad downstream O2 sensor, exhaust leak, bad ETC, bad ECM, etc.

When the converter was replaced the last time around - was it an OEM one, or an aftermarket one? Easiest way to tell is by price - if it was under a couple of hundred - probably aftermarket. If that is the case, it is possible that the cat converter is going bad again - as those aftermarket ones only have a fraction of the elements found in an OEM one.

Also possible that the downstream O2 sensor is dying or not reading correctly. OEM Denso sensors used? Aftermarket ones have a reputation for reading incorrectly, or sometimes the ECM will freakout immediately.

With that amount of mileage, and that you've already changed out the cat and the O2 sensors - I'm heavily leaning toward the next common issue - exhaust leak. Could be leaking at the manifold, downpipe, or right behind the cat - exhaust gaskets, donuts are also suspect to leak. Especially if you live in an area that salts the roads regularly in the winter time - I'd definitely look this a possibility. Best way to get a look is to get it on a lift and visually check the exhaust system, look for tell-tale signs of the exhaust leaking (soot).

Thanks for the reply. Yes, the catalytic converter was OEM as were the oxygen sensors. At the same time that I had the sensors installed, they also replaced the exhuast manifold. The CEL kept coming back until the converter was changed. But it should not have failed again this soon. I will take the car in tomorrow and hopefully they will get it figured out without having to change every part again. I'll ask them to check for leaks. I do live in an area where there is a lot of salt on the roads in bad weather. Probably not important, but the CEL came out after I had driven about 60 miles on the freeway in a very heavy rain.

Let us know what they find out.

There is a good chances that since you had the catalytic converter, O2 sensors and the exhaust manifold replaced - very likely, in the course or removal and replacement, those gaskets (even brand new ones) might have gotten damaged during install. After a short amount of time, they can leak - causing the downstream O2 sensor (post-cat) to read incorrectly and cause the P0420 code popping up.

Road salt doesn't help either - the OEM exhaust system is pretty tough, can have a significant amount of surface corrosion before perforation happens, but it can and does happen.

The shop confirmed the PO420 code. They did a quick check and didn't see anything, but said they had had several people come in recently with the code and that it could be falsely set by the cold temperatures. They reset the light for now and will wait to see if it comes back. From my previous experiences, my guess is it will. I do believe that the other times I have had this code, it has been during winter when the weather is very changeable. I've also always wondered about the change to the winter blend of gas. If cold temperatures are the problem, I will have to live with the light for a few months.

Yeah, that code is annoying, but not anything that would be detrimental to the performance of the car. Just indicates that the downstream O2 sensor waveform is not changing as expected.

Fish, do you agree that the cold weather (or heavy rain, fog I was driving in) would cause the CEL to falsely pop up? Any thoughts on winter gas blend? Thanks for your help!

I haven't seen it with the P0420 code, but I do get cold weather related CELs for the EVAP system. I've gotten into the habit of smogging my car earlier than later, as those pesky EVAP codes seem to pop up more often in the wintertime / cold weather. During the rest of the year / moderate temps - zero CELs.

Winter gas blends "shouldn't" mess with the overall A/F mix enough to cause the downstream O2 sensor to get confused. Though some of the additives they blend in, like butane, could spike the octane enough to cause problems that the ECM cannot correct for.

The heavy rain could have, splashed up and shorted the leads on the downstream O2 sensor - that would do it.

I was able to get two weeks without the CEL coming back. While driving in foggy, rainy weather again yesterday, it popped up again. After having been through this so many times, I was expecting it. Had the code read at Autozone to make sure, and of course, it is P0420. So now I must decide if I will get a fourth catalytic converter or live with the light. I've been searching on the internet and it's amazing how many people with this model car has had this same problem. This has been a great car (other than this CEL problem), but because of this, I would not buy another toyota. I'm going to see if I can get shop to look over everything real good to make sure there are no other problems that would cause the code, but in the past, I had multiple other parts changed and nothing worked until a new converter was put on. At 176,000 miles, I don't know if I should or not.

Not just your generation, also affects the 8th gen as well. Also a major issue with lots of European imports, BMW seeming to lead that.

Unfortunately, much of this may have nothing to do with the actual catalytic converter and more with ECM sensitivity and threshold setting. There was a TSB that dealt with excessive sulphur odor from this generation (more with the gasoline than the car) - Toyota implemented a ECM flash that helped reduce that odor by modifying the A/F and timing map. An interesting consequence is that owners that had the reoccurring P0420 code, never had it come back after the reflash.

If this will help in your case - that is unknown. But it definitely points to potential possibility. Gone through 3 cats - assuming these were all OEM catalytic converters and not aftermarket, if aftermarket - actually sounds about right.

The next common cause is a bad or dying downstream O2 sensor. This sensor's output is used to compared the upstream waveform to the downstream one. If they look too "similar" - it will set a DTC and you get the P0420 bad cat efficiency code.

Owners have gotten the car smogged via the tailpipe and didn't notice anything wrong with the cat's efficiency. A simple fix is use the sparkplug defouler trick. Basically moving the downstream O2 sensor slightly away from the exhaust stream causes its outputted valves to fall into the correct threshold. Some owners get creative and use a downstream O2 simulator to trick the ECM.

Since yours seem to be weather dependent, could be a connection issue to the downstream O2 sensor or exhaust leak close to the sensor. Just because the code says "bad catalytic converter" does not always mean that cat is faulty. If the technician didn't diagnose the issue further than reading the code - they didn't really fix anything.

Fish, thanks, I always appreciate your input. I did have the sulfur smell pretty bad in my car in the beginning. In fact, I had it until I had the first converter replaced and after that it never came back. I initially took it to dealer that time and they said I need the new cat and showed me the TSB for the sulfur and said they might also need to do a reflash after the cat was put on. They were going to charge 1,200 for the cat and also charge for the flashing if it needed it. I opted to wait for awhile and got other opinions. Was able to get cat replaced for less (still expensive) a few months later and the sulfur smell was gone.

I'm going to wait until I get some time to take it in for a good inspection. As I've said, the car has been great except for this problem and I know there could be much worse problems.

No, the ECU wasn't recalibrated. I didn't know anything about the TSB until I took the car to the dealer after the first instance of the PO420 (110,000 miles). They only wanted to replace the cat ($1,200) and then see if the the flashing needed to be done. I opted to go to another shop to have the cat installed. That converter lasted about 20,000 miles and the current one is at about 42,000 miles.

The recalibration will give it more tolerance between the upstream and downstream O2 sensor signal input, to lessen the P0420 code tendency... My cat is fine with about 255,000 kms (158,000 miles) on it, and I still never get code P0420. I did replace both 02 sensors at 185,000 kms (115,000 miles) with new Denso's when my downstream O2 sensor's heater circuit went out.

Does your engine consume much oil?



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