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What causes a catalytic converter to go bad?

What makes a catalytic converter to go bad?

Catalytic converters are designed to last the life of the vehicle. So when there’s a problem owners want to know what causes a catalytic converter to go bad? Or, if they’ve recently replaced a catalytic converter and now the new one is bad they want to know what made the new catalytic converter go bad?

Well people, it’s all your fault

A properly maintained engine puts out exhaust that any catalytic converter can handle. Think of it this way; it’s really nothing more than an incinerator that uses any unburned gas and oxygen left after combustion as a fuel to create enough heat to finish the combustion process. The metals in the catalytic converter react with the unburned fuel and oxygen, causing a catalytic reaction that produces heat. And yes, it’s the heat that does the job.

The precious metals are deposited on a ceramic honeycomb called a “brick” and the exhaust flow through the tiny passages (as many as 1,200 cells per inch on some newer vehicles) forcing the exhaust to interact with the metals and react.

Unfortunately, too much unburned fuel can cause this incinerator to overheat and that’s what melts the ceramic brick and destroys a catalytic converter. And, if the engine develops a coolant leak, leaking fuel injectors, misfires due to worn spark plugs, wires, or coils, or excessive oil consumption, those items can also contribute to overheating, plugging, and honeycomb contamination. In other words, everything that can kill a catalytic converter is caused by poor engine maintenance or neglect of a problem.

Let’s look at each contributing factor in what causes a catalytic converter to go bad?

First, let’s talk about exhaust gas recirculation (EGR). The air we breathe is mostly nitrogen. When combustion temperatures are high, oxides of nitrogen form in the combustion chamber and flow out with other exhaust gasses. Oxides of Nitrogen are a major contributor to smog. To reduce NOx, car makers try to lower the combustion temperatures by recirculation exhaust and adding to the mix of fresh air and new fuel. They can’t add exhaust gas at idle because it would choke off combustion. But they can add it without any ill effect at higher engine RPMS.

So the engine management system monitors engine RPMs, pedal position, and speed and determines when to add exhaust and how much to add. Then it commands an EGR valve to open and allow exhaust flow into the intake manifold. Because the exhaust contain carbon contaminants, and because the exhaust is hot, carbon can force in the exhaust passage and on the EGR Valve itself, causing it to block exhaust flow, clog the passage completely, or prevent the EGR valve from closing. Each of these conditions would set a trouble code and light the Check Engine light. If you were to respond to the Check Engine light and see an EGR related code, you might be tempted to think, “Well, it’s just emissions related and my car runs fine. I’ll ignore it.” Well guys, you just made a decision to cause the early demise of your catalytic converter. Because without proper EGR, combustion temperatures run too high. That not only increases NOx emissions, but also overheats the catalytic converter, causing damage to the ceramic brick. A new EGR valve costs about $100. Replacement is easy, taking less than one hour of shop time. A new catalytic converter costs about $1,000. And, you STILL have to replace the EGR valve or you’ll destroy the new catalytic converter as well.

Worn spark plugs, wires, and ignition coils also damage catalytic converters. Any time a spark doesn’t occur when it’s supposed to, or doesn’t happen at all, the air/fuel mixture in that cylinder isn’t burned properly and the mixture flows into the catalytic converter. As I said earlier, excess oxygen and fuel cause a catalytic converter to overheat and fail. A severe misfire will set off a blinking Check Engine light. At that point, you’re already in a danger zone. Your misfire has been occurring all along and it’s finally reached a point where the catalytic converter is being damaged. Spark plugs cost less than $10 each. Spark plugs wires cost less than $100/set. If you don’t replace them on time, you’ll damage a $1,000 catalytic converter.

Coolant leaks caused by gasket failures are a sure way to contaminate a catalytic converter. The chemicals in coolant coat the surface of the honeycomb brick, rendering it useless. If you have an internal coolant leak where coolant seeps into the combustion chamber (white smoke from the tailpipe is a symptom of burning coolant), and you don’t repair it immediately, you’ll destroy the catalytic converter. If you simply replace the converter, you’ll destroy the new one as well.

If your engine consumers oil, the excess oil flowing into the exhaust stream will burn and form carbon deposits on the honeycomb, eventually clogging it. Worn valve stems, cracked valve stem seals, worn piston rings are all contributors to oil consumption. Ignore them and you’ll destroy your catalytic converter and the replacement catalytic converter as well. Once you reach about 1-quart of oil consumption every 1,000, you’re at risk of damaging the catalytic converter.

Silicone sealant also damages catalytic converters. NEVER use it anywhere on the air intake or on the exhaust. It will prevent the metals in the catalytic converter from reacting with the exhaust.

Impact is a huge issue for converters. The ceramic brick is supported in the stainless steel housing by a high temperature matting. If you run over a curb or parking lot speed bump too fast and dent the catalytic converter, the dent can contact the brick, causing it to vibrate and disintegrate.

Overuse of cleaners like fuel injector cleaner and Seafoam. Sure, you think you’re doing your engine a favor by using those cleaners. But where do you think all that crud goes? As soon as it clears the exhaust manifold it goes right into the cells of the honeycomb brick.

People complain that catalytic converter manufacturers routinely deny their warranty claims. But the manufacturers aren’t stupid. They examine every converter returned under warranty. They check for impact damage on the case. They look for metal discoloration that would indicate overheating. Overheated metal appears grey and shows signs of rust. They look for plugged cells in the honeycomb brick—yes, your engine problem caused the catalytic converter to plug up. It didn’t come from the factory that way.

They cut the converter apart and test for coolant, oil, and silicone contamination. They also check the condition of the matting to see if it shows any sign of impact damage.

©. 2015 Rick Muscoplat



Posted on by Rick Muscoplat

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