The P0420 code signifies a problem with the catalytic converter or more serious repairs. If the code shows in the car’s PCM, then it means it has identified a problem; in this case, with the catalytic converter.
The catalytic converter reduces emissions that exit the car through its exhaust pipe and removes harmful pollutants that were created during combustion in the engine.
The driver is not under any risk of harm related to the P0420 code, but they should have the problem fixed sooner rather than later to avoid further issues, as failing an emissions test.
Our opinion is that you shouldn’t drive your car until you have fixed the P0420 code. There are some repairs related to the P0420 code that you can do yourself, but many of the more extensive repairs will need to be handled by a mechanic.
What Does P0420 Code Mean?
The P0420 code signals low catalyst system efficiency, and that means oxygen levels are below thresholds (bank 1) most likely due to issues with your car’s exhaust or fuel systems. It’s not necessarily a straightforward problem to diagnose.
The catalytic converter is part of the vehicle’s exhaust system and it works by oxidizing the remaining unburned fuel and carbon monoxide CO from the combustion process into carbon dioxide (CO2) and water (H2O), while also reducing nitrogen oxides (NOx) to nitrogen.
The converter uses two oxygen sensors, one upstream and one downstream.
If the upstream oxygen sensor is functional, the readings should fluctuate (the readings should not be the same) from when the car is at operating temperature and running in a closed loop.
If the downstream oxygen sensor is functional and there is nothing wrong with the catalytic converter, the readings should remain steady (fluctuate up and down but remaining steady around a value) during the same scenario.
When the through-put oxygen sensors have the same readings, it can indicate the catalytic converter is not doing its job.
When the voltage of the downstream oxygen sensor drops, and begins to fluctuate like the upstream oxygen sensor, it means oxygen levels are too high, and the Powertrain Control Module (PCM) will trigger the P0420 code, and the check engine light will come on to alert you that you have a catalyst problem.
Key Summary:
- P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1).”
- This trouble code is triggered when the on-board computer detects that bank 1 of the catalytic converter is underperforming.
- A faulty catalytic converter (usually due to overheating and contamination) is the most common reason why this code is logged.
- Once a P0420 code is detected, your vehicle can exhibit symptoms like an illuminated check engine light and a failed emissions test.
Most Common Causes of The P0420 Code
There may be one or more of these causes with a P0420 code:
- Using leaded fuel when unleaded was specified (unlikely).
- Faulty or damaged O2/ oxygen sensor.
- Oxygen sensor (HO2S) wiring/connection is damaged or defective.
- Faulty engine coolant temperature sensor.
- Damaged or leaking exhaust manifold/catalytic converter/muffler/exhaust pipe.
- Faulty or ineffective catalytic converter (likely).
- Retarded spark timing.
- The O2 sensors before and after the converter are reading too similarly.
- Fuel pressure issue or a leaking fuel injector.
- Cylinder misfire.
- Oil contamination.
4 Common Symptoms of A P0420 Code
A bad catalytic converter usually doesn’t show any signs or symptoms. That said, if you suspect that you may have problems in the dubious dangers of your exhaust system, here are a few of the most common symptoms.
The following list will allow you to detect a possible broken catalytic converter:
1. Loss of Power
If your vehicle hesitates or noticeably struggles to gain power while you’re driving after warm-up, you may have a failing catalytic converter.
Fuel pressure can become uneven because of the worn out converter, which could affect the way your car responds when you press on the accelerator. You may also note jerky movements while you are driving or flat-out stall-outs.
2. Not Able to Accelerate or Speed Up
As we mentioned in Point 1, a bad catalytic converter will cause exhaust to build up in the exhaust pipe and a potential loss in power, which also affects its performance.
A bad cat. converter is evident if you notice your car didn’t accelerate when you hit 30-40 mph, or you begin to see a decrease in fuel efficiency, too.
3. Bad Smell from the Exhaust Pipe
This bad smell occurs from having too much oxygen in a catalytic converter, which causes excess sulfur in the fuel tank, thus causing the bad smell from the exhaust system.
4. Check Engine Light (CEL) Is ON
The most common sign of any engine code diagnosis is the check engine light on your dashboard. Keep in mind that the check engine light does not necessarily mean DTC P0420, but it might possibly be one of the fault codes keeping your engine light illuminated.
- You might also see a failed emissions test for a reason where a bad catalytic converter is involved.
- If your check engine light is illuminated, you would want to prevent more extensive damage and have the problem diagnosed ASAP!
Can I drive with the P0420 code?
In the short term, it is usually acceptable to drive with a P0420 code. The driver is at no risk due to this code. This code can cause harm to your engine and exhaust system if you drive it for an extended period.
If you notice this code, you do not need to get a tow truck, at least not yet; however, if this code comes up, you should do your best to rectify the problem as soon as practical. You could have more noticeable issues with how your car performs.
This is due to your exhaust system not performing correctly. It could be more problematic if the other codes indicate too much fuel in your air/fuel mixture.
Look for any other codes, particularly P0171, P0172, P0174, or P0175. These codes could indicate that your engine has the wrong balance in fuel-to-air ratio.
How do I diagnose P0420?
Four important steps must take place in the diagnosis of P0420:
1. Confirm there are no other codes
You should deal with any other codes first, since these codes could be the cause of the P0420 error. An OBD2 scanner can reveal if there are any other codes. If there are any codes like P0300, P0171, etc, they could be the source.
Only after correcting these codes can you replace your catalytic converter. If you replace the catalytic converter before correcting the other code, your new catalytic converter will fail again.
2. Clear the code and drive to see if it returns
You can also check to see if the code is random by clearing the code and driving.
Sometimes a sensor in your car sees abnormal patterns. Once that abnormal pattern has been seen over a long period of time, there is either a problem with the sensor or a problem with the vehicle. The check engine light turns on as a result.
So, even if the condition may resolve itself after a few periods, you should try to see if your check engine light turns on. Go ahead and turn off your check engine light. Then keep driving for about 15 minutes to see if the check engine light comes back on.
If it doesn’t come back on, then it probably was a temporary error, and you can leave it alone. But be careful. If the check engine light comes back on quickly after you turned it off and throws the same code, it is probably a problem you need to figure out and repair.
3. Check for leaks in your exhaust system
If, after clearing the code and driving, you’ve confirmed the P0420 is still there, you should do a check of the exhaust system. You should inspect the exhaust system, particularly what is done before the catalytic converter. Check for leaks, rust, holes, etc.
Check the gaskets, exhaust pipes, and exhaust manifolds. You have to fix any leaks or damage you find.
4. If you have to replace the catalytic converter, check the downstream O2 sensor
Start your vehicle and make sure the engine is at normal operating conditions. Then use a digital multimeter to check the voltage of the downstream O2 sensor.
The downstream O2 sensor should be approximately 0.45 volts, indicating the catalytic converter is working properly. If the voltage fluctuates between 0.1 and 0.9 volts, then the catalytic converter must be replaced. This final voltage indicates that the catalyst section in the Catalytic Converter is not working properly.
Common Diagnosis Mistakes
The most common misdiagnosis associated with fixing the P0420 engine code (or possibly storing any P0420 code) is replacing parts without fully diagnosing the issue first. Many people will simply change the O2 sensor or oxygen filter when addressing the OBD II trouble code P0420.
But the cause of the code could be totally different, such as a bad spark plug. So ensure you’ve considered and properly diagnosed all potential issues before settling on a repair.
It probably isn’t safe to assume that a faulty upstream oxygen sensor or downstream oxygen sensor was simply assumed.
How Serious Is Engine Fault Code P0420?
The P0420 issue is not always a safety hazard when driving. In fact, the defect is unlikely to cause serious drivability or reliability problems.
However, the engine may be entering more noxious, or hazardous, pollutants into the environment as the catalyst system on bank 1 is not operating properly. This, combined with the check engine light, will most likely make you ineligible for a successful emissions inspection.
As with anything concerning your engine, the best course of action is to diagnose and repair the P0420 issue as soon as possible. So, we are not saying that you should simply let these problems go unaddressed for weeks or months.
But it is probably safe to continue driving with engine fault code P0420. Most importantly, you should evaluate driving with any engine trouble code P0420, as warranted. You know your car and how it feels the best! And if it feels bad, more dangerous, or you have other engine codes, severe symptoms, etc., then play it safe!
What Repairs Can Fix the P0420 Code?
Since the P0420 code can be due to several factors involving the exhaust system, there is no single solution. Before repairing P0420 you must ensure you have properly diagnosed the reason the code has been set.
- Replace muffler or repair leak in muffler.
- Replace exhaust manifold or repair leak in exhaust manifold.
- Replace exhaust pipe or repair leak in exhaust pipe.
- Replace catalytic converter (most common)
- Replace engine coolant temperature sensor.
- Replace front or rear oxygen sensor.
- Repair or replace damaged wiring to oxygen sensors
- Repair or replace oxygen sensor connectors
- Replace or repair fuel injector(s) that are leaking.
- Diagnose and repair any misfiring problems.
- Diagnose and repair any other related trouble codes stored by the Power Control Module (PCM)
How Much Does P0420 Code Repair Cost?
Repair for DTC P0420 cost can vary widely depending on the diagnosis; if you have a minor exhaust leak, then possibly even just welding the holes could be nearly free. Other P0420 related repairs could be pricey. In general, repairing P0420 could cost anywhere from $10 to $2,000 or more.
Below are the ranges of prices if you had a mechanic perform some of the common repairs:
- Air fuel sensor $200-$300
- Oxygen sensor replacement $275-$500
- Catalytic converter $400-$2400
- A leak in exhaust 100-200 (if welded)
If you are a do-it-yourselfer, it will be much less expensive, but you risk doing more damage, if you are not experienced. The most important thing to remember before you fix P0420, is to accurately diagnostic the cause of the code.
If you clear the code and it does not return during driving, then you can spend no money addressing the problem.
If there is a small leak in the exhaust system which you can weld, then you will not spend much money. Conversely, if you do need to replace the catalytic converter then it will be an expensive repair.
We strongly encourage that if you need to replace the catalytic converter, that you replace it with an original manufacturer branded unit (i.e. bought from a dealership). One thing to remember is that emissions parts usually are covered against repairs longer than regular warranty.
So for example, if you have a brand new vehicle but it is out of your bumper to bumper warranty, that does not negate any warranty if it is the type of problem that manufacturers have extended warranties on.
Many manufacturers even have a five years unlimited mileage warranty. This may be worth looking into.
FAQs.
Which O2 sensor causes P0420?
When the oxygen sensors upstream and downstream detect that the catalytic converter is not functioning properly, a P0420 code is triggered. In essence, the downstream sensor recognizes that some of the oxygen entering the converter is not being burned.
Will a P0420 code cause a misfire?
In a nutshell, P0420 can and will make a car lose its ability to accelerate and/or experience multiple misfires or selective cylinder misfires. Your congested exhaust is most likely the source of your issue, which will result in excessive backpressure and a loss of power.
Where is P0420 Bank 1?
When your car’s computer receives the P0420 code, it means the Bank 1 catalytic converter isn’t performing as it should. The side of the engine that contains the first cylinder is referred to as “Bank 1.” Bank 2 is located on the engine’s opposing side.
Why am I still getting a P0420 code after I replaced my catalytic converter?
O2 sensors before and after the catalyst as well as the catalyst itself may be to blame for the code P0420. You might have an exhaust leak at the manifold or pipes if both the O2 sensors and the catalyst were replaced.
How long does it take for P0420 to clear?
Reset the code and continue driving for around 15 minutes. Check if the code is returned. If not, try it again a few times to be certain. When the DTC P0420 appears, a real issue exists.