Rick's Free Auto Repair Advice

How to fix Code P0420

What is code P0420 Catalyst System Efficiency Below Threshold

When attach a trouble code reader and come up with a P0420, your first assumption may be that you need a new catalytic converter, simply because the words catalytic converter are mentioned in the code. After the tests you may indeed discover that the catalytic converter is bad. But first you have to understand how the air/fuel and emissions systems work.

How the air and fuel mixtures are calculated

The powertrain control module (PCM) receives inputs from the engine coolant temp sensor (ECT), the mass airflow sensor (MAF), and the throttle position sensor (TPS). At that point the PCM knows air temp, engine temp, air density and how much power you’re asking for. Then it calculates how much fuel is required to get you the power you need AND complete burn so you don’t pollute the air. To check on how well it did calculating the air/fuel mixture, it checks the readings from upstream oxygen sensors (O2). If the readings shows the exhaust is rich, the PCM will cut back on fuel. If the reading is too lean, it’ll add fuel.

That sounds easy, right? BUT, since you’re constantly changing what you’re asking from the engine by  going up and down hills, accelerating and decelerating, the computer is CONSTANTLY recalculating the air/fuel mixture and the upstream O2 sensor is CONSTANTLY reporting how well the PCM did. In other words, the PCM is constantly overshooting and undershooting the air/fuel mixture and trying to narrow the gap between the two. Since the PCM is constantly playing “catch-up” to recalculate.

What the catalytic converter does

If the PCM overshoots and provides too much fuel, that extra fuel gets burned off in the catalytic converter. If there’s too much oxygen left over after combustion, the catalytic converter stores that oxygen to use to help burn off the next round of excess fuel. If the catalytic converter is doing its job properly, the downstream O2 sensor, located AFTER the cat converter, should rarely switch between rich and lean. If it does toggle between rich and lean, the sweep should be fairly mild.

Code P0420 represents a problem

However, if the downstream O2 sensor starts sweeping between rich and lean, or constantly reads a rich or lean condition, that’s an indication to the PCM that the cat converter isn’t doing its job of cleaning up the PCM’s mistakes. The code P0420 can be caused by an engine problem that’s pouring too much fuel into the catalytic converter, excess fuel that the PCM can’t control, a vacuum leak that’s causing excess air to enter the catalytic converter, OR, a damaged catalytic converter

What goes wrong with a cat converter?

P0420, P0430, catalytic converter

Cut-away of a typical catalytic converter

Catalytic converters have a ceramic honeycomb inside and the surfaces contain a coating of precious metals like platinum, Palladium, Rhodium, and other non-precious metals. Those compounds react with unburned fuel in an oxidation/heating process that completes the burn and neutralizes oxides of nitrogen (a component of smog). It’s similar in concept to a municipal garbage burner where they add treat the smoke with extra heat to reduce emissions.

P0420, P0430, catalytic converter, check engine light flashing

Cat Converter showing honeycomb in good condition

However, if too much unburned fuel enters the cat converter, the oxidation process creates too much heat and the converter starts a runaway reaction. If that continues, it causes a meltdown

P0420, P0430, flashing check engine light

Melted ceramic honeycomb. This is a destroyed converter caused by ignoring engine problems.

where the ceramic honeycomb actually self destructs. Once that happens, the cat converter is toast. The point here is that cat converters don’t die on their own, they’re always murdered. Excess fuel, bad valve stem seals that leak oil into the combustion chamber, coolant leaks that send coolant into the exhaust stream—all of those engine problems can kill a cat converter. If you replace a converter without fixing the underlying problem, you’ll just have to replace it again.

How to diagnose code P0420?

Always start by connecting a scan tool that’s capable of displaying live data. And don’t whine to me that you don’t have one of those. Scan tools have dropped in price and you can get one with live data for less than $100.

Here’s the lecture: If you want to work on your own car, you must have the right tools. In the old days you had to own a tach/dwell meter and a timing light. So shut up and stop complaining about how you have to buy a scan tool. Either invest in the right tools or take it to the shop and pay them $100 to do a scan.

Now, connect the scan tool, start the engine and let it run until the tool shows that it’s in CLOSED LOOP. That means it’s no longer running off factory programming and is taking information from all the sensors and making its own calculations. Then, have a friend drive the vehicle while you monitor FUEL TRIM readings. Depending on your vehicle, you may be able to monitor both short and long-term fuel trim. Fuel trim is the amount of EXTRA fuel the PCM has to add to keep the engine running properly. In an ideal world with a brand new engine, the short term fuel trim should be 0. But in an older vehicle, a short-term fuel trim reading of 10% is acceptable. The maximum short term fuel trim is 25%. When the PCM exceeds that, it starts boosting long-term fuel trim, so check both. In other words, a low short term fuel trim with a higher long term fuel trim means you have an air/fuel problem.

What causes excessive fuel trim?

The number one villain is a vacuum leak after the MAF sensor has measured how much air is coming into the engine. That causes the upstream O2 sensor to see a continually lean exhaust and it adds fuel to compensate. Plugged fuel injectors, a bad fuel pressure regulator, and a bad fuel pump can also cause a lean condition.

If the fuel trims are in the normal range, next check the upstream O2 sensors. A healthy O2 sensor should switch between rich and lean at least 8-10 times in 10 seconds. Many times a scan tool won’t react that quickly and that may mislead you. So check the scan tool to see if it displays CROSS COUNTS. That metric actually does the counting for you. Professionals use a scope to check for cross counts. If you find that the upstream O2 sensor ISN”T switching that often, you have a “lazy” sensor. That causes the PCM to react too slowly, forcing the cat converter to see too much fuel, then nothing. That causes the downstream sensor to switch between rich and lean and cause a P0420 or P0430 code. Bottom line: if the upstream sensors are lazy, they’ll cause the downstream sensors to switch and make you think the cat converter is bad. Ignore the problem and the cat converter WILL go bad from all the extra fuel. So replace the lazy upstream sensors, clear the codes, and complete another drive cycle. Click here to understand drive cycles.
CLUE: If the upstream O2 and downstream O2 are both switching rapidly, the cat converter isn’t doing its job. A properly operating system should show rapidly switching upstream sensor and a downstream sensor that rarely switches.

If the fuel trims are ok, and you’ve replaced the O2 sensors, confirmed that the fuel pressure is correct, that you have no excessive blow-by, worn valve stem seals, or coolant leaks, and you still have code P0420, you’ll have to replace the cat converter. Don’t buy a cheap one. It’ll set off the check engine light.

© 2012 Rick Muscoplat

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Posted on by Rick Muscoplat


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