P0422 & P0432 (Main Catalyst Efficiency Below Threshold)

The rear O2 Sensors compare the data with the front O2 Sensors and have figured out the catalytic converter (cat) is no longer doing a efficient job.

I got this code on a 2001 Jeep Wrangler (TJ) and it started coming on and off by itself, then went away altogether.

A bad cat could probably damage the rear O2 sensor.
If a car is operated with bad O2 sensors it can lead to premature failure of the cat. For example, if I understand the operation of these systems correctly, a bad O2 sensor can lead the computer system to deliver more fuel when less is warranted, which can lead to the catalytic converter getting too hot and failing (as well as lowering gas milage and increasing emissions). That is one of the reasons it is usually a good idea not to ignore check engine codes.

About Catalytic Converters:

A three-way catalytic converter has three simultaneous tasks:

  1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2
  2. Oxidation of (toxic) carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
  3. Oxidation of unburnt carcinogenic hydrocarbons (HC) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O
These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point. This is between 14.8 and 14.9 parts air to 1 part fuel, by weight, for gasoline (the ratio for LPG, natural gas and ethanol fuels is slightly different, requiring modified fuel system settings when using those fuels)

See: Exhaust System

Problem could be the canister control valve (CCV), an oxygen sensor, or catalytic converter

The most common fix for this problem is to simply replace the Catalytic Converter. There is the off chance the rear O2 Sensor is failing to read correctly.

It might be a bad catalytic converter (cat), but be sure everything else is good before spending money on that. Plugs, wires and O2 sensors should be checked. A cloged fuel filter.

A bad O2 sensor should give an error code P0130-P0167

A variety of conditions may cause the catalyst to overheat (heat deactivation) and potentially to melt down. Some factors that can cause this are:

  • lubricating oil in the exhaust system (caused by engine wear, or by damaged rings or valves)
  • An engine misfire or ignition failure (causing unburnt fuel to enter the exhaust)
  • A cracked exhaust valve (again, causing unburnt fuel in the exhaust)
  • An engine misfire or ignition failure (causing unburnt fuel to enter the exhaust)
Overly rich fuel mixtures are not usually a problem - there is too little unused oxygen for the exothermic reaction to be large enough to cause damage. A slightly lean of stoichiometric (quantitative relationship [usually a ratio] between reactants and products in a chemical reaction) mix is far more dangerous, as the oxygen level is elevated, allowing a very large exotherm (generation of heat).

I had a friend with a 1999 VW who got a P0422. The garage said the engine was running lean and there was an oil leak and to use high octane gas, which solved the problem.

A leak upstream of the cat is going to lean out the mixture, making the car run richer than it should, which might be giving a false indication (but probably not).

Causes of converter failures: Fouling, clogging, melt-down and breakage of the ceramic substrate inside a converter are common conditions that can cause problems. Sometimes an indication that a converter is clogged is that you don't go any faster when you push the gas pedal down. Plugging is usually the end result of a melt-down, which occurs because the converter gets too hot. This happens because the engine is dumping unburned fuel into the exhaust. The excess fuel lights off inside the converter and sends temperatures soaring. If it gets hot enough, the ceramic substrate that carries the catalyst melts. The unburned fuel may be getting into the exhaust because of a bad spark plug or valve, but an overly rich air/fuel mixture is another possibility. In older carbureted engines, a heavy or misadjusted carburetor float may be the underlying cause. But on newer engines with "feedback" carburetion or electronic fuel injection, the engine may not be going into "closed loop" (the normal mode where the computer regulates the air/fuel mixture to minimize emissions). A bad oxygen sensor or coolant sensor may be giving the computer bogus information. A sluggish or dead O2 sensor will make the computer think the exhaust is running lean, so the computer will try to compensate by making the fuel mixture rich. A coolant sensor that always indicates a cold engine will also keep the system in open loop, which means a steady diet of excess fuel. But it might not be the sensor's fault. A thermostat that's stuck open or is too cold for the application can prevent the engine from reaching its normal operating temperature. So if your converter has failed and needs to be replaced, the engine should be diagnosed for any underlying problems before the new converter is installed. Another cause of converter clogging and contamination is excessive oil consumption. Worn valve guides or seals can allow oil to be sucked into the engine's combustion chambers. The same goes for worn or damaged rings or cylinders. Oil can form a great deal of carbon, and metals present in the oil can contaminate the catalyst. A compression check or leak-down test will tell you if the rings are leaking, while a fluttering vacuum gauge needle will help you identify worn valve guides.
Source: How can I tell if my catalytic converter is working properly? at IndiaCar.com

Diagnostic proceedure for Kia:
Connect computer
Look at front ane rear O2 sensor graphs.
Replace Cat if O2 Sensor graphs indicate it is deteriorated.

If Front O2 Sensor voltages are consistently high, or freeze frame data shows an excessively high negative long fuel trim, check fuel pressures and injectors.

If Front O2 Sensor voltages are consistently low, or freeze frame data shows an excessively high positive long fuel trim, check fuel pressures and injectors (same as above)
Also check all vacuum hoses and intake for vacuum leaks. Make sure there are no exhaust leaks which could dilute the rear O2 sensor readings.

Forum Comments:
Q: I continue to get a P0422 code. The engine runs great. I just replaced during a tune up: plugs, wires, cap, rotor and both O2 sensors. I also replaced the catalytic converter and exhaust. The catalytic converter came from a place called Autohaus AZ and was smaller but otherwise similar to the original. The exhaust is a stainless performance exhaust with a larger diameter than the original.
A: have you checked the MAF (Mass Air Flow) sensor? ANy exhaust leaks ahead of the 02 sensor or converter, like say the manifold?

Q: My car lost power about a week ago, I could not get it going above 40m/h. Took it to the mechanic who said that the catalytic converter and the "j pipe"? was completely shot. Gave me a quote for over $4000 to repair which effectively writes off the car (2000 Hunday Sonata).

A: $4000 seems a little high. Shop around for price for the repairs you need. Most aftermarket converters don't compare well in quality or effectiveness to the factory converters, but they're also typically much less expensive.

Just as a check, I ran the numbers to check the dealer list price for the converters and front pipe (parts only) for a 2000 Sonata four cylinder and came up with this:

Front converter: $636.37 Front pipe and rear converter: $820 or $1202, depending on which emissions system you have.

if you have less than 80,000 miles on the car, the converter should be covered under warranty. I believe it is not legal to install an aftermarket converter while the car is still within the warranty period.
I just called VW at 1 800 893 5298... They informed me the warranty on the catalytic converter in the US is 10 years and 120000 miles..

Exhaust System
How Muffler Exaust System Works at 2CarPros.com
How can I tell if my catalytic converter is working properly? at IndiaCar.com
Catalytic Converter at Wikipedia

Catalytic Converter at autorepair.about.com
Discussion at MercedesShop.com/shopforum/

last updated 11 Dec 2007