Rick's Free Auto Repair Advice

P0420 caused by bad O2 sensor?

P0420 can it be caused by a bad O2 sensor?

Does a bad oxygen sensor cause a P0420 code?

Answer: Rarely, but it depends on the vehicle

On many pre-1996 vehicles the upstream oxygen sensor uses a “narrow band” type sensor, and post 1996 vehicles often use this type of sensor as the downsteam sensors.

These sensors compare the amount of oxygen in the exhaust to the amount of oxygen in the outside air. It obtains and ambient sample through one of the wires (between the copper and the insulation). If the engine has dripped oil onto the electrical connector or dirt/debris has coated the wires, the sensor will give inaccurate data to the ECM.

So it’s possible a bad narrow band sensor can cause a P0420 or P0430, but it’s rare. Checking the condition of the O2 sensor connector and wires should be part of every diagnostic when encountering a cat converter code.

In addition, a lazy oxygen sensor can cause the ECM to command overly rich/lean air/fuel mixtures and prolonged operation in that mode can damage the catalytic converter.

BUT, the ECM is also programmed to set an O2 sensor trouble code when it detects long periods of slow O2 sensor response. The parameters were much more lenient in the first generation O2 sensors, but later model vehicles use air fuel sensors and those would set an O2 sensor code long before faulty air/fuel reading could damage the cat converter.

So, while it’s technically possible for a lazy O2 sensor to cause catalytic converter failure, it’s really not probably.

You also have to understand that to condemn a catalytic converter, the ECM uses readings from the O2 sensor. If the sensors were bad, it couldn’t set the P0420.

Let’s understand exactly what a P0420 code is. Here’s the definition:

P0420 Catalyst System Efficiency Below Threshold (Bank 1).

What the code is telling you is that the catalytic converter has failed an efficiency test. The ECM tests the catalytic converter to gauge its efficiency when certain conditions are met. A new catalytic converter starts out at close to 99% efficiency and quickly tapers off to around 95% efficient. But after that, the converter can’t drop any more than a few percentage points. Once the converter drops to 92% the ECM will set a P0420 or P0430 code. The newer Low Emission Vehicle (LEV) vehicles have even tighter requirements for efficiency.

How does the ECM know the catalytic converter is operating below efficiency?

Once certain operating parameters are met, the ECM will rapidly change the air/fuel mixture from rich to lean and back again. A lean mixture will fill the catalytic converter with unburned oxygen, while the rich mixture will add unburned fuel. The upstream oxygen sensor will detect the swing between rich and lean and the switching rate must match the rate at which the ECM commanded the change. If it does, the ECM knows the upstream sensor is good.

Next, it monitors the downstream oxygen sensor. During the lean phase, the converter stores the excess oxygen. During the rich phase, the converter uses the oxygen to burn off the excess fuel. The result should be an exhaust stream that is neither rich nor lean. In other words, the downstream sensor should show little to no movement, proving that the converter has done its job and burned off the excess fuel.

If the converter has lost efficiency, it won’t be able to store as much oxygen, so it can’t burn off the excess fuel. The downstream sensor will see a swing from rich to lean and back again, in time with the upstream sensor. That’s the ECM’s clue that the converter isn’t working and it sets a P0420 code or a P0430 code.

What’s the lesson here? A bad oxygen sensor can’t cause a P0420 code because the ECM NEEDS both the upstream and downstream sensors to operate properly IN ORDER TO DETERMINE the converter isn’t working!

If either the upstream or downstream sensor doesn’t switch in cadence with the ECM’s changing air/fuel ratios, the ECM will set OXYGEN sensor codes, not catalytic converter codes!

Is there anything else that can cause a P0420 code beside a bad converter?

YES! An exhaust leak. That’s why a full exhaust system inspection should be the first thing you check if you have a P0420 code.

• 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.

exhaust leaks

Check 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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