Rick's Free Auto Repair Advice

Diagnose a catalytic converter P0420, P0430

How to diagnose a catalytic converter P0420, P0430 the proper way

How catalytic converters fail

Overheating, melting, or a broken substrate can cause a P0420 or P0430

Overheating is caused by feed gases from the engine that exceed normal limits. Excessive fuel or oil in the exhaust can cause the catalytic converter to overheat and melt. Or, the overheating can cause the matting inside the catalytic converter shell to deteriorate, allowing the substrate to bang around and break.

Catalytic converter poisoning can cause a P0420 or P0430

Excessive oil in the exhaust or and internal coolant leak like a head gasket failure can coat the small passages inside the catalytic converter, causing it to fail.

Structural damage can cause a P0420 or P0430

Driving over parking lot curbs, speed bumps or striking road debris can dent the catalytic converter and damage the substrate.

In addition, impacts can damage the flanges and exhaust gaskets that connect the catalytic converter to the exhaust system. Stripped O2 sensor threads or damaged O2 sensor bungs can also cause those codes.

Understand what P0420 or P0430 codes actually mean

P0420 Catalyst System Efficiency Below Threshold (Bank 1)
P0430 Catalyst System Efficiency Below Threshold (Bank 2)

Both codes are telling you that the catalytic converter isn’t doing its job. Neither of these trouble codes say “Dude, replace O2 sensor.” With very few exceptions (on some Subarus, a bad O2 sensor will NOT set a P0420 or P0430. Quite the opposite, the ECM uses the upstream and downstream sensor to test the catalytic converter’s efficiency. If the O2 sensors were bad, the ECM wouldn’t even be able to complete the cat converter test and the ECM would fail the O2 sensors and set L2 sensor codes instead.

Step-by-step approach to diagnosing a P0420 or P0430

1) Start by checking for a manufacturer’s technical service bulletin

Manufacturers often issue service bulletins to correct software issues that result in false P0420 or P0430 codes. It makes no sense to go through all the tests listed below or replace parts only to find out that there’s a software update that fixes the problem.

Alldatadiy.com and eautorepair.net sell subsctiptions to shop manuals and service bulletins. Some public libraries offer free access to these services. Check the service bulletins first!

2) Start with a visual inspection

Check the catalytic converter for signs of overheating, impacts, dents and leaks. Overheating will show as a bluish discoloration. Check the integrity of the O2 sensor and the wiring harnesses to the sensors.

If you see dents or impact damage, chances are high that the substrate has been damaged. If you see signs of overheating, the cat converter has most likely been damaged by feed gases that exceed allowable limits. You must fix the root problem before replacing the cat or you’ll just kill the replacement.

overheated catalytic converter

3) Perform a rattle test on the catalytic converter

Using a rubber mallet, tap along the entire length of the catalytic converter listening for rattling sounds that would indicate a broken substrate.

4) Check for exhaust leaks with an exhaust system pressure test

Set up your shop vac in blow mode. Connect the hose to the tail pipe. Wrap with duct tape to seal the hose to the tail pipe. Turn on the vac to pressurize the exhaust system.

Using a spray bottle with dishwashing detergent and water, spray each connection in the exhaust. If you see bubbles, fix the leak and then repeat the test.

5) Use an infra-red thermometer to perform a temperature differential test on the catalytic converter

Exhaust gases entering the converter should be cooler than the gases exiting the converter if the converter is working properly.

This test will give you a rough idea of the converter’s performance, but it is not fool proof! Different pipe wall thicknesses, pipe corrosion, along with different heat transfer rates may cause inaccurate results. So the EPA recommends that this test be used only to prove that a catalyst is good.

Start the engine and get it to full operating temperature (heater off). Point your infra-red thermometer at the pipe upstream from the catalytic converter weld ring (the point where the inlet pipe is welded to the converter body). Note the temperature.

Next, point the thermometer at the weld ring at the outlet of the catalytic converter. Note the temperature.

Calculate the difference between the readings. If the outlet reading is higher than the inlet reading, then some conversion is taking place. Only older vehicles you may notice a 150° difference between inlet and outlet. But the difference is much lower on late model vehicles with well tubed engine. On those vehicles, a 20°F difference can indicate a fully functional catalytic converter.

If you see no difference in temperature, the catalytic converter is most likely dead.

6) Hook up a scan tool and check fuel trim live data

High fuel trims (above 10%) indicate a serious problem that must be fixed before attempting to replace a catalytic converter. Check for vacuum leaks and intake air duct leaks that would cause the ECM to add fuel.

If you have high fuel trims, find out why before replacing anything!

7) Check for a clogged catalytic converter by conducting an exhaust backpressure test

If you have a lack of power along with the P0420 or P0430, you may have a melted/clogged catalytic converter. A plugged cat converter can cause a lack of power because it restricts exhaust flow. The symptoms usually show up as a car that’ll start and run but lack power at higher speeds.

You’ll need an inexpensive vacuum gauge from any exhaust backpressure gaugeauto parts store (usually less than $15).

Remove the downstream oxygen sensor and connect the backpressure gauge into the O2 bung.  Tighen to O2 torque specs.

Start the engine and note the reading at idle. Then increase engine RPM to 2500 and note the pressure reading.

Backpressure at idle on most vehicles is around 1-psi. At 2500 RPM, the pressure is around 3-psi.

If your readings are high the blockage probably exists downstream of the test point, which typically means it is in the muffler or resonator.

If the back pressure is okay at this location, move the gauge to the upstream O2 sensor location and repeat the test. If the back pressure is high at this point, the converter is causing the issue.

If pressure at the upstream O2 sensor is in the normal range, then the problem is upstream of the catalytic converter.

8) Conduct a volumetric efficiency test

A volumetric efficiency test tells you how the engine is breathing and can help determine if there’s an airflow problem due to restrictions in the intake or exhaust or faulty sensors

A Volumetric Efficiency (VE) measures the engine’s current ability to move air versus it’s theoretical ability based on engine design.

You’ll need a scan tool with live data and a recording function to conduct the test

Watch this video by auto trainer David Hobbs to learn how to conduct a VE test

How a catalytic converter works to clean exhaust

In simple terms a catalytic converter is an incinerator designed to burn off any extra, gas oil or coolant that makes its way into the exhaust stream. It also removes oxides of Nitrogen, a major component in smog. It also removes excess fuel, oil and coolant, as well as carbon monoxide

The catalytic converter is a ceramic honeycomb that coated with atoms of platinum (Pt), palladium (Pd), and rhodium (Rh).

Combustion creates Oxide of Nitrogen (NOx)

Nitrogen is in the air we breath. In that form, it’s not a pollutant. However, during the combustion process, high combustion temperatures create an endothermic reaction that combines can cause oxygen and nitrogen to form nitric oxide (NO), nitrogen dioxide (NO2) or nitrious oxide (N2O2).

When nitric oxide and nitrogen dioxide (NO2) combine with sunlight it transforms NO2 into NO, depending on the intensity of the sun. Ozone present in the air also converts NO2 to NO.

When NOx, NO and NO2 combine with volatile organic compounts from aerosol spray cans and emissions of hydrocarbons from industrial manufacturing plants they into the forms photochemical smog. The smog contains increased levels of ozone and peroxide compounds known at peroxyacetyl nitrate which is dangerous to persons with any type of lung disease or asthma. Worse yet, it CAUSES lung disease in people work or exercise outdoors.

Finally, NO2 can also combine with hydrogen and oxygen to Nitric acid (HNO3). Nitric acid contributes to acid rain

The combustion process also produces hydrocarbons (HC) and poisonous carbon monoxide (CO)

So the catalytic converter’s first stage converts NO and NOx into N2 and O2

The first stage of the catalytic converter is the reduction stage. It’s job is to convert nitrious  oxide (N2O2) and nitric oxide (NO) into nitrogen and nitrogen dioxide (NO2). To accomplish that, catalytic converter makes use platinum and rhodium. As the NO and N2O2 enter the catalytic converter, the nitrogen molecules are strongly attracted to the metals and and less attracted to the oxygen molecules. So the oxygen molecules debond and continue to flow into the 2nd stage of the catalytic converter.

Meanwhile, as more nitrogen molecules enter the catalytic converter, the bond with  each other more than they bond to the the platinum and rhodium. Once they combine, they debond from the metals and flow out the exhaust as plain nitrogen (N2).

In other words, what came into the catalytic converter as harmful nitrogen oxide gases (NO and NO2), now leave as harmless nitrogen (N2) and oxygen (O2).

The first stage is called the reduction stage simply because it reduces NO and NO2 to N and O.

The second stage is the oxidation stage and its job is convert carbon monoxide and HC into CO2 and H2O

So hydrocarbons HC (unburned fuel), N2 and carbon monoxide (CO) enter the second stage of the catalytic converter.

There, the single oxygen molecules bond with platinum and palladium. In other words, stage 2 stores oxygen. When HC enters, the O2 and CO, along with heat from the exhaust react with the HC to oxide it and render it into CO2 and H2O.

The third stage cleans up any remaining NOx

How can you damage a catalytic converter?

Feed it unburned fuel

Problems with your ignition system or fuel system can cause too much fuel to enter the catalytic converter. A lean misfire (too little gas for the amount of air causes combustion to end early), causes the unburned fuel to enter the catalytic converter. A rich misfire on the other hand, (too much gas for the amount of oxygen in the cylinder), also causes unburned fuel to enter the catalytic covnerter.

When too much gas enters the second, or oxidation stage, it causes a runaway reaction that results in extremely high heat—heat that’s so high it can actually melt the ceramic honeycomb structure.

What causes unburned fuel to enter the catalytic converter?

Ignition misfires
Poor air/fuel mixtures caused by clogged fuel injectors or improper fuel pressure
Worn piston rings or valves and prevent proper combustion.

Feed it coolant

Internal head gasket leaks can cause engine coolant to enter the combustion chamber where it’s burned. Unburned coolant damages the oxygen sensor and degrades the catalytic converter.

Feed it oil

Worn piston rings and valves can cause too much oil to be burned in the combustion chamber. All that extra oil goes into the catalytic converter, causing it to overheat and melt.

Impact damage that shatters the ceramic honeycomb and causes the converter to clog



Start the engine and let it warm up. Then connect the vacuum gauge

vacuum gauge, check engine vacuum, catalytic converter

Vacuum gauge with fittings

to any vacuum line going into the intake manifold. You should see a steady reading between 18-22-in. The needle should be steady and not bouncing. Bouncing is an indication of a bad valve.

Next, slowly increase the speed to around 2,500-RPMs and hold it there. The needle on the gauge will drop as you open the throttle and then stabilize. If the vacuum is low or drops after stabilizing, that’s a sign you’ve developed backpressure in the exhaust system. That backpressure could be either the catalytic converter or the muffler. So disconnect the muffler and recheck. If you still have a low or decreasing reading, the converter is bad.

You can also measure backpressure directly with a backpressure gauge. Remove the upstream oxygen sensor and install the backpressure gauge fitting in its place. Then start the engine. You should see 1-psi or less of backpressure at idle and less than 4-psi when you snap the throttle open.

©, 2013 Rick Muscoplat

Posted on by Rick Muscoplat

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