Rick's Free Auto Repair Advice

Diagnose a catalytic converter

How to diagnose a catalytic converter to make sure it’s dead

A p0420 or P0430 trouble code usually is an indication of a bad catalytic converter. But do you know for sure that the catalytic converter is dead? Catalytic converter do fail. But sometimes things are happening upstream that can cause a P0420 or P0430 and they don’t necessarily mean the catalytic converter is bad.

This post deals with how to check out those upstream possiblities as well as checking the actual condition of the catalytic converter. This is one in a series of articles about catalytic converters. To read more, click on these posts

Catalytic converter problems

Po420 catalyst below efficiency

First, understand what P0420 or P0430 actual mean

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

P0430 Catalyst System Efficiency Below Threshold (Bank 2)

This is basically the same bode that says the catalytic converter isn’t doing its job. If you have a 4-cylinder engine, you’ll only get a P0420 codes. On a V-style engine you may be one or both codes.

Neither of these trouble codes say “Dude, replace the catalytic converter.” All they say is that the catalytic converters have failed an efficiency 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.

I continually see people complain about lack of power in their cars and they wonder if they have a plugged catalytic converter. Don’t just assume you need a new one. You must diagnose a 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. A bad fuel pump or clogged fuel filter can also cause these symptoms. If you’ve replaced the fuel filter and checked the fuel pressure, you may want to read this article on how to diagnose a catalytic converter. Also consider a bum converter if your car is overheating and the cooling system checks out.

You’ll need an inexpensive vacuum gauge from any auto parts store (usually less than $15). Start by checking the condition of the converter. If it’s blue, that’s a dead giveaway that it has overheated and most likely melted the inner ceramic core. And check for dents from impacts. That can also destroy the core. If there’s no sign of physical damage, check engine vacuum.

Overheated catalytic converter

Overheated catalytic converter

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

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

Custom Wordpress Website created by Wizzy Wig Web Design, Minneapolis MN