Rick's Free Auto Repair Advice

P0420 P0421 P0422 P0423 P0424 P0430 P0431 P0432 P0433 P0434

Fix code P0420 P0421 P0422 P0423 P0424 P0430 P0431 P0432 P0433 P0434

Some blog readers have been asking if they really have to replace a $1,200 catalytic converter

P0420, P0421, P0422, P0423, P0424, P0430, P0431, P0432, P0433, P0434

Catalytic Converter

just because they’re seeing these codes. Perhaps not. But before I dig into what may be causing the problem, let’s do a quick explanation of exactly what a catalytic converter does and how it may fail.

Inside the catalytic converter you’ll find a honeycomb coated with rare-earth metals like Rhodium, cerium, platinum, and palladium. These metals react with oxygen, fuel, oil, to reduce the amount of carbon dioxide and oxides of nitrogen released into the atmosphere.

To check on the efficiency of the catalytic converter, manufacturers install an oxygen sensor in front of and behind the converter. The front oxygen sensor is very busy. It constantly senses changes in the exhaust. In fact, a properly operating oxygen sensor senses switches from lean to rich mixtures as often as 10-times per second. That data gets fed back to the computer to alter air/fuel mixtures going into the engine. If the catalyst is operating properly the post-cat sensor should see VERY FEW changes.

Now let’s talk about how it all works.

Hot exhaust gas warms the metals in the cat converter to about 475-575 degrees F. At that point the cat converter is referred to as “lit up.” In other words, that’s hot enough for it to start doing it’s job of burning off excess fuel and oil. As soon as the cat converter see excess fuel, the catalytic operation begins and the operating temperature rises to 1,200- 1,500 degrees F. The converter can easily handle those temps. But if the engine is misfiring and dumping excess fuel or the engine is worn and dumping excess oil, the converter can overheat to upwards of 2,500 degrees. At that point the ceramic honeycomb begins to melt and self destruct. Please re-read the previous sentence. The point is cat converters self destruct BECAUSE of engine misfires and worn engine conditions that dump too much fuel and oil. They don not fail from normal use.

There are a few other things that can kill a cat converter. First, if you think your car is a real screaming machine and you’re dumb enough to run racing oil in it, the extra friction modifiers in racing oil (zinc and phosphorous) will kill the converter. If you do an engine repair and use too much RTV sealant or the sealant isn’t “sensor safe” that too can kill a cat converter. Finally, a coolant leak can foul oxygen sensors. At that point they’re blind and with no fuel control, you’ve set up the conditions for converter burn out.

Now let’s focus on the codes

P0420 Catalyst System Efficiency Below Threshold (Bank 1)
P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)
P0422 Main Catalyst Efficiency Below Threshold (Bank 1)
P0423 Heated Catalyst Efficiency Below Threshold (Bank 1)
P0424 Heated Catalyst Temperature Below Threshold (Bank 1)
P0430 Catalyst System Efficiency Below Threshold (Bank 2)
P0431 Warm Up Catalyst Efficiency Below Threshold (Bank 2)
P0432 Main Catalyst Efficiency Below Threshold (Bank 2)
P0433 Heated Catalyst Efficiency Below Threshold (Bank 2)
P0434 Heated Catalyst Temperature Below Threshold (Bank 2

Every control module runs a catalyst efficiency “monitor.” In other words, it runs a cat converter test. First, it determines that the engine is up to operating temperature. To do that it looks at the engine coolant temp sensor. Next, it looks at the upstream oxygen sensors to

P0420, P0421, P0422, P0423, P0424, P0430, P0431, P0432, P0433, P0434

Oxygen Sensor

ensure they’re switching from lean to rich (or vice versa) at least 10-times per second. If the upstream sensor is working and the cat converter is working, then the downstream sensors (after the car) should be seeing any variations. That would mean the cat converter is doing it’s job of converting fuel, oil, carbon dioxide, and oxides of nitrogen into harmless compounds. To sum up, the computer is looking for a rapidly changing upstream sensor and a very quiet downstream sensor. If the downstream sensor is active and exceeds the standards by 1.5 times, the computer determines that the catalyst efficiency is below threshold. It then sets a P0420 trouble code. The big mistake DIYers make in not checking other codes.

For example, if the upstream O2 sensor is failing (called a lazy sensor
because it’s not switching often enough) the computer may be dumping too much fuel into the mixture. That extra fuel may cause the POST O2 sensor to vary too much. That’ll set a cat converter code and trick you into thinking the converter is bad. So first address any OTHER codes present before you even think of swapping out the cat converter.

To check for a lazy sensor (one that’s failing enough to cause a problem but not serious enough to set a trouble code) most technicians use a scope. But you can use a scan tool –one that includes a readout for “cross counts” (that’s the technical term of when an oxygen sensor switches from lean to rich and vice versa). You should also check Short and Long Term Fuel Trim. If those numbers are 0-10%, you’re ok. But if they’re between 11-25%, the computer is adding too much fuel and that’s an indication of a serious problem. You can see how important it is to own a serious scan tool if you really want to diagnose the root cause of the problem. Buy one here.

Scan tool, Actron, OTC,

Live Data Scan Tool

However, if you don’t own a scan tool and want to solve the problem yourself, start by replacing the UPSTREAM O2 sensors. That may sound counterintuitive since the codes actually indicate a problem with the downstream sensors. But remember, it the UPSTREAM sensors that control air/fuel mixtures. If they’re going bad, the air/fuel mixture will be screwed up and that will interfere with the downstream sensor readings.

If you’ve replaced both upstream sensors and the code still persist, then it’s time to replace the downstream sensors. Still have a code? Well, now you’ve got one more option. Force the cat converter into “clean out” mode. This involves dumping so much extra fuel into the exhaust that it forces the cat converter to overheat and possibly burn off the contamination. Yes, this procedure and also destroy the converter—but you’re already at that point and don’t have much more to lose. To perform this procedure, disconnect a spark plug wire or coil pack wire and run the engine at 1,500 to 2,000 RPM for about 5-10 mins. The extra fuel from the misfiring cylinder should raise cat converter temps to almost 2,000 degrees. After 10 minutes, reconnect the spark plug or coil pack wire and take it for a hard drive at freeway speeds. That should force enough of the burned off carbon buildup out of the converter. If it works, you just saved yourself $1,200. If it doesn’t, you lost nothing but some extra gas.


© 2012 Rick Muscoplat


Posted on by Rick Muscoplat

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