Intermittent p0420 - Cat efficiency issues

Po420 is a failing catalytic converter and it's not uncommon at 150K.

 

No.

You might try replacing the air/fuel ratio sensor first, to see if that will help. If not then the catalytic converter will need to be replaced.

A failing catalytic converter has no impact on fuel economy or engine performance (unless the cc is beginning to clog up, in which case engine power would be restricted.)

 

Replacing the Air Fuel Ratio sensor is a waste of money. This is a newer car and has a post cat sensor.
It monitors the cat for efficiency loss.

 

Yes, I understand that.

As the engine ECU varies the air/fuel mixture, it compares the voltage output provided by the upstream air/fuel ratio sensor, with the voltage output provided by the downstream oxygen sensor. The engine ECU expects the upstream sensor voltage output to vary widely while the downstream sensor voltage output should be fairly constant which proves the catalytic converter is functioning correctly in converting HC to CO2 and H2O.

If the upstream sensor is marginal then its voltage readings will not vary as widely as a new sensor. Hence the installation of a new sensor may help the engine ECU to decide the catalytic converter is acceptable.

Especially since the OP has pointed out that the issue is intermittent, that provides further hope that replacing the air/fuel ratio sensor may allow the OP to defer replacement of the catalytic converter.

 

Not quite! An A/F sensor doesn't have the same signal as an Ho2s.
The post sensor input is the determining factor for P0420.

 

I am very interested in this subject as well as I have been getting the P0420 code intermittently.

After clearing the code it might come on after a few days, or may take many weeks. It never comes back on within the same day of clearing it.

This week I have cleaned the MAF/TB and replaced the spark plugs. I am going to run only Chevron gas and plan to put a bottle of Techron through the system as well here soon.

Is there a way to check the O2 sensors while leaving them hooked up without buying a OBD II scanner?

I would rather not remove any sensors to do a blow torch bench test and I'm not keen on the idea of splicing into the signal wires. I suppose it could be possible to run a small jumper line in between the male/female plug junction. Feel free to flame me is that is a bad idea.

I'm simply looking to make intelligent choices in troubleshooting this code rather than making my wallet lighter by blindly ordering parts.

Any input is greatly appreciated as I'm always looking to learn!

 

Replace the downstream O2 sensor first, it is much cheaper than the A/F ratio sensor.

 

Thanks for your input!

 

You need to use a capable scanner to read active A/F and post cat sensor data. It's a pretty fool proof system and most (99%) P0420's are cat failures. As the precious metals on the substrate degrade along with the converters ability to store oxygen, it causes the post cat sensor to detect the AF ratio changes in the exhaust stream. The ECM detects it based on the post cat feedback and sets the DTC. Replacing the post cat sensor won't fix it. It's a total waste of time and money. Everyone tires these fixes to get around paying for a cat, but it's a fruitless effort. Don't waste your time. Don't buy the cheap FleaBay universal converters. They won't work. Buy an OEM unit from Toyota and save yourself the headaches.

 

Yes, it is true that the downstream oxygen sensor is less expensive. However, replacing it does not help if the intent is to try to get around the P0420 code without replacing the cc.

As the oxygen sensor deteriorates, it tends to be less responsive to changes in air/fuel ratio, which is exactly what you want here. You want the downstream sensor voltage output to be slow to change.

If you replace the downstream sensor, the new part will be more responsive, hence will produce greater voltage fluctuations.

I agree with this, once the OP has decided to invest in a cc replacement. However this requires the OP to step up to paying $1,650 MSRP (or ~$1,236 plus shipping if he buys from one of the dealers selling discounted parts over the web) plus the associated labor cost.
2006 TOYOTA PRIUS Parts - AutoNation Toyota Gulf Freeway Parts

Hence the interest in trying to defer that four-digit repair price.

 

Techron works great on the fuel system and will clean off carbon on the intake valves and combustion chamber.
It won't fix a P0420

 

Intermittent CEL for P0420 usually means a failing downstream O2 sensor.

 

More urban myth and misinformation is more like it

 

A dirty MAF will report skewed data, but if it was causing a rich condition to the point of converter damage the system would set a P0172 or the cat would be glowing red hot.
It won't report a P0420 immediately. It will take at least 3 drive cycles before the system sets the DTC. Most of the monitors need to run and set before it fails it. This is why it appears to be random.

 

Get a capable scan tool and look at the data. It's the only way to know what's taking place. To many interweb idea's always lead to wasted money replacing parts that aren't the problem. I see it everyday and it always cost more to try and save money by doing it that way. If you don't have or can't get access to the proper equipment, find a qualified shop to do the diagnostic work. It's always worth the money to know what's wrong. Don't pay for guesswork either. Shops they say things like we thing it could be, or we need to replace this first are just guessing and wasting your time and money. The dealers are really good at the dart game of automotive diagnostics