Rick's Free Auto Repair Advice

P0420 — Diagnose catalytic converter and oxygen sensor

P0420 — Is it the catalytic converter or the oxygen sensor?

What does P0420 mean?

Here’s the official definition of a P0420 trouble code: P0420 Catalyst System Efficiency Below Threshold (Bank 1).

What the code is telling you is that the catalytic converter has failed an efficiency test. In 99.5% of the cases, this code means that the catalytic converter itself is bad and must be replaced. However, I see questions all the time on auto forums asking if a new oxygen sensor will fix the problem. The answer is—almost never.

What’s driving the issue is that everyone wants the cause of the P0420 to be a bad oxygen sensor so they don’t have to replace the expensive catalytic converter. However, there are tests you can perform to find out for sure whether it’s the catalytic converter or the oxygen sensor.

Tests to perform if you have a P0420 or P0430

Diagnose and fix misfire codes first

Understand that engine misfires can cause a P0420 or P0430 code because misfires load the converter with fuel. So always correct misfire codes first before you begin to diagnose a Po420 or P0430 code.

Check for exhaust leaks ahead of the catalytic converter

Exhaust leaks can skew the pre and post converter O2 sensor readings and cause either of the converter codes. Raise the vehicle and look for signs of soot around every exhaust pipe connection. If you have an open space, start the engine and use an automotive stethoscope to listen for exhaust leaks.

Check for open/closed-loop operation

Use your scan tool to make sure the engine is going into closed loop operation quickly. If it isn’t, fix that problem first.

Check fuel trim numbers

Normal fuel trims run from 0 to 10%. If you see a short or long term fuel trim value that exceeds that, find out why the computer is adding more fuel. The root cause can be unmetered air (air duct leak), dirty MAF sensor, vacuum leak or clogged fuel injector.

Perform a cooling system pressure test

A failing head gasket can force coolant into the combustion chamber, causing a rich condition in the exhaust and contamination of the O2 sensors. If the cooling system pressure shows that the cooling system is fine, proceed with other tests.

To diagnose a P0420 you’ll need a scan tool with live data

Well, kinda. You don’t have to test anything. You could just throw a new upstream or downstream oxygen sensor in and see if it works. But I’m telling you, you’re most likely wasting your money.

That’s because before the computer sets a P0420 trouble code, it has ALREADY tested your oxygen sensors. However, if you really want to do more testing, there’s no way around this, you need to see live data to determine if the oxygen sensors are providing the right data AND you need to perform a volumetric efficiency (VE) test to see if the catalytic converters are plugged. VE determines how well the engine is breathing. See this post on smartphone and laptop OBCII Scanners

You also need to determine if a misfire caused the cats to go bad so you can fix the underlying problem and not damage the new catalytic converters.

How to check for misfires

This is really simple: misfires kill catalytic converters. So the first thing to check if for current and history misfire codes P0300 through P0308. If you’ve recently cleared your trouble codes, you’ll have wiped out this information—bad move on your part. But you can still get some useful misfire information by using Mode $06

If you don’t have any misfire codes and your OBDII scanner is capable of reading Mode $06, try looking for misfire codes there. Locate the parameter I.D. (PIDS) for each cylinder and see how many misfires have accumulated in Mode $06. If you find misfires, correct the misfire problem before you go any further. Learn more about how to use Mode $06 here.

How to check fuel trims

Locate short and long term fuel trims on your OBDII scanner. Fuel trims is the addition or subtraction of fuel made by the ECM in order to achieve proper emissions. The addition or subtraction is determined by the ECM’s reading of the upstream oxygen sensor. Most vehicles are capable of adding or subtracting up to 25% more or less fuel before the ECM sets a fuel trim related trouble code.

Generally speaking, fuel trims on older vehicles should vary by no more than 10%. On newer vehicles, the fuel trims should be no more than 5%. However, it’s not enough to check fuel trims while idling. You must check them in all modes of operation. So take the vehicle for a test drive and watch fuel trims when accelerating, cruising, highway speeds and declaration. If the fuel trims stay within 5% or 10%, then fuel isn’t the issue.

How to check oxygen sensor readings

The ECM is constantly changing the air/fuel mixture to achieve the right proportion for each driving condition. So the upstream oxygen sensor will change rapidly from rich to lean. The downstream oxygen sensor, on the other hand, should show very little movement. It’s testing the catalytic converter to see if it’s storing oxygen during rich events and then using the oxygen during lean events.

During a P0420 readiness monitor test, the ECM will purposely and rapidly command a very rich and very lean mixture to force the downstream sensor to read the excess fuel or oxygen. Then it will stop the excessive air/fuel and watch the downstream sensor again, looking for a steady reading. If it sees that behavior, it knows the catalytic converter and the oxygen sensors are operating properly. However, if the oxygen sensor doesn’t switch rapidly during the test, the ECM will set a downstream O2 sensor trouble code.  NOTE: This is why a P0420 trouble code is almost never a bad oxygen sensor — because the ECM has already tested the downstream sensor. If it isn’t responding properly, it will set a trouble code. The same applies to the upstream sensor. If it’s not switching rapidly, the ECM will set an O2 sensor related trouble code

If the upstream and downstream oxygens sensors are both switching from rich to lean, that’s the sign of a dead catalytic converter.

Here’s where you could have an O2 sensor related P0420

An exhaust system leak really screws up the P0420 testing routing. An exhaust leak near the upstream sensor can cause the downstream sensor to read a steady lean condition. The ECM would interpret that as a bad catalytic converter. In that case, your OBDII scanner should be reading a lean 0.1v reading from the downstream sensor. You can add propane fuel to the exhaust to force the reading up to prove that the downstream sensor is good. If it returns to lean after you remove the propane, you’ve got an exhaust leak.

Test drive

A properly operating catalytic converter and muffler system and an engine with good fuel trims and no misfires should have good acceleration at wide open throttle. So take the vehicle to a safe area and put the pedal to the metal. If you notice a lack of performance, you should suspect exhaust obstruction. That’s when you perform an volumetric efficiency test.

Conduct a volumetric efficiency test

Set your OBDII scanner to record intake air temperature, RPM, MAF, MAP, BARO and fuel trims. Starting from a slow roll put the pedal to the metal until the vehicle makes the 1-2 shift. Then head home and enter the peak values into a volumetric efficiency calculator. Intake air temperature should decrease during acceleration. An increase in intake air temperature during acceleration is automatically a sign of a plugged catalytic converter or muffler, regardless of the VE calculator results.

Enter the data on a volumetric efficiency calculator like this. A low reading indicates an engine breathing problem. A P0420 and a breathing problem is a dead ringer for a bad catalytic converter

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat

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