Fix P0420: Mechanic’s solutions that WORKED!

FOLLOW THIS GUIDE if you have a P0420 code. I will explain what the code means, how to diagnose it, and all that backed up by 100+ fixes case study, where I dug up old service reports from real mechanics sharing their experience of fixing P0420 on various different cars.

P0420 quick overview

Meaning	P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Is it serious?	Yes, this code is serious because it indicates that your catalytic converter is not functioning properly, which can lead to increased emissions and potential damage to other engine components.
Possible causes	-Failing catalytic converter on Bank 1 -Faulty oxygen sensor (pre or post-cat) on Bank 1 -Exhaust or intake leaks -Engine running rich (fuel system issues) -Engine burning oil or coolant -Faulty PCM needing an update or reprogramming
How to diagnose?	Read OBD2 Codes: Perform a full scan to check for additional codes that may indicate underlying issues. Compare O2 Sensor Data: Analyze live data from O2 sensors on both banks to identify discrepancies. Check Catalytic Converter Temperature: Use a laser thermometer to measure temperature differences at the cat’s inlet and outlet. Perform Exhaust Pressure Test: Use a pressure gauge to check for back pressure in the exhaust system. Inspect for Leaks: Check for exhaust or intake leaks that could affect emissions readings.

P0420 Fix study: 101 repairs from mechanics

I went through hundreds of old service reports directly from mechanics that fixed the P0420 problem on different car models. Here are the most common solutions.

Replace Catalytic converter (59%)

Most mechanics often tell you that the P0420 means a bad catalytic converter in 99% of cases. It was actually 59% in the study I made, but it is still the most common solution to repair this problem.

I was able to find 3 cases where the cheaper aftermarket converter fixed the problem however there were more cases of OEM replacement because the aftermarket didn’t work. The study is backing up what mechanics are saying: Aftermarket Catalytic Converters don’t work!

Replace oxygen sensor (15%)

The second most common repair was replacing the oxygen sensor. One repair was done by repairing the short in the O2 sensor’s electrical circuit, but there was one other interesting fix.

The technician looked at the O2 sensor’s live data graphing which was very odd and after unplugging sensor 1, the code for sensor 2 was stored in the ECU. The connectors for O2 sensors were somehow swapped with each other and after switching them, the car was fixed.

Other (26%)

I also found several other fixes, the most commonly air/exhaust leaks and ECM reflash. It can be worth calling the dealership if there is any ECM update available, it can solve the problem without replacing an expensive Catalytic converter.

Fixing P0420 on a Diesel was mostly done by replacing the DEF injector in the exhaust.

P0420 Fix + diagnostic process for different cars

Here are several reported Fixes with a brief explanation of the diagnostic process, and they are followed by a table of 101 reports that worked for the displayed car model.

2015 Ford Transit-350 HD XLT 3.2L MIL Lamp on (Diesel)

When you run into a Catalytic converter issue, it will be the gasoline engine most of the time. But here is at least one example of a diesel engine.

Problem: Check engine light

P0420- Catalyst Efficiency Below Threshold
P24a0- Closed Loop Particulate Filter Regen Control at limit – temp too low
P2463- Particulate Filter Restriction

Diagnosing: The technician was able to find unusual behavior of the DEF injector (exhaust injector) and replaced the nozzle which seemed to fix the problem, but the car was back after a few thousand miles with the same issue.

During the second inspection, he found metal particles in the fuel filter.

FIX: Replace the fuel pump

2002 GMC Envoy SLE 4.2L with p0420

Problem: Check engine light (P0420)
Diagnosing: The car came in with P0420, the technician looked at the rear O2 sensor data and decided to replace the Catalytic Converter because the voltage would flatline. I think this was slightly misdiagnosed because while the front O2 sensor’s voltage should fluctuate on the graph up and down, the downstream sensor should flatline which confirms the good condition of CAT.

FIX: The CAT was replaced, but the code didn’t disappear. The technician also noticed a slightly rougher idling and decided to clean the throttle body. It is possible that the CAT wouldn’t even have to be replaced, maybe cleaning the throttle was the only thing to fix the problem.

1995 Subaru Legacy L 2.2L with p0420

Problem: Check engine light (P0420), Hesitation, Cuts out, Stall
Diagnosing: The customer even experienced a car dying at a red light after driving for a longer period of time. The technician tried to replace cam/crank sensors based on what he found worked for other Subarus but without luck. After diagnosing a few parts with DVOM without finding anything, he tried to hook the oscilloscope to MAF (mass airflow sensor) and found that it dropped voltage to set the code.

FIX: Change MAF (mass airflow sensor)

1997 Mazda Protege DX 1.5L with p0420

Problem: Check engine light (P0420)
Diagnosing: Fist step was replacing the exhaust manifold after noticing the crack in it. The code came back after a few hundred miles, the rear cat was checked and confirmed to be good (this car has 2 cats). The rear O2 sensor was changed, but it didn’t help either then advice came from an experienced Mazda Technician.

FIX: The advice from Mazda Tech was to replace the front CAT, which FIXED the problem.

2001 Nissan Pathfinder LE 3.5L with p0420

Problem: Check engine light (P0420), Rear O2 sensor (P0158)
Diagnosing: Since the code for the rear O2 sensor was stored, the obvious way was to change them. The technician decided to change both rear O2 sensors because the code was sensor and the code for cat were on different banks.

FIX: Replacing both rear O2 sensors worked.

1998 Toyota Sienna LE 3.0L with p0420

Problem: Poor Fuel Economy, MIL Lamp on (P0420)
Diagnosing: The technician noticed unusual live data reading from the upstream O2 sensor (before CAT), and tried to replace it with an aftermarket sensor. The car returned after a while, again with unusual live data reading from the pre-Cat O2 sensor. The technician replaced the sensor again but with the OEM Toyota part.

FIX: Toyota doesn’t use classic oxygen sensors but wideband air/fuel ratio sensors. The aftermarket part didn’t work, but the new OEM sensor FIXED the problem.

2004 Dodge Grand Caravan SXT 3.8L with p0420

Problem: MIL Lamp on (P0420)
Diagnosing: The Technician’s job wasn’t easy on this one as he got this car with P0420, however, the car already went through service in the dealership where both cats, all oxygen sensors, temperature sensors, and even spark plugs were replaced. He found out that something keeps clogging the CATs, so had to inspect the engine to find what is clogging it.

FIX: The technician was able to find damaged valve seals leaking oil inside some of the cylinder’s combustion chambers by looking at the spark plugs (some clean, some dirty from oil). Fixed by replacing the seals, and CATS one more time.

1999 Honda Civic EX 1.6L with p0420 code

Problem: MIL Lamp on (P0420)
Diagnosing: The technician tried to clean the fuel injectors because of found a clogged fuel filter and replace the front O2 sensor because he found the data unusual. The code kept coming back and he decided to replace the catalytic converter.

FIX: The problem is fixed by replacing CAT and both oxygen sensors.

2008 Subaru Outback 2.5L with p0420

Problem: MIL Lamp on (P0420)
Diagnosing: The data on the oxygen sensor look unusual so he decided to change both of them, clear the codes and return the car to the customer. The car is back after a few days with the same code. The technician replaced the valve cover gaskets and catalytic converter.

FIX: The problem is fixed by the second repair, maybe the CAT was bad, or the damaged valve cover gaskets were affecting the air/fuel ratio to set this code.

2003 Chevrolet Silverado 1500 5.3L

Problem: MIL Lamp on (P0420, P0101)
Diagnosing: The technician replaced the fuel pump and all O2 sensors without solving the problem. Then the car was taken to the dealership since it is still under warranty for CAT, which was replaced.

FIX: New catalytic converters solved the problem.

2005 Honda Pilot EX 3.5L

Problem: MIL Lamp on (P0420)
Diagnosing: The technician tried to consult the dealership which advised the replacement of all 3 catalytic converters, but then decide to follow the advice found in Aldata.

FIX: Bank 1 CAT replacement fixed the issue, other cats weren’t replaced.

101 solutions that fixed the p0420

I dug up 101 closed cases all reported by mechanics who fixed the problem. Here are the cars and reported fixes that worked. Click on “Show more” to expand the table.