P0420 – Catalytic converter system, bank 1 -efficiency below threshold (Mitsubishi)


By Reinier (Contact Me)
Last Updated 2021-01-06
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P0420 Catalytic converter system, bank 1 -efficiency below threshold Catalytic converter, wiring, HO2S 2

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What Does Code P0420 Mean?

Special note on trouble code P0420 and Mitsubishi vehicles: While DTC P0420 is a generic code that affects all OBD II compliant vehicles, the most common causes of this code sometimes vary between most vehicle makes. This article will therefore deal with code P0420 as it applies specifically to Mitsubishi vehicles.

OBD II fault code P0420 is a generic trouble code that is defined as “Catalytic converter system, bank 1 – efficiency below threshold” and is set when the PCM (Powertrain Control Module) detects a condition in which the efficiency of the catalytic converter that serves Bank 1 has fallen below a minimum allowable threshold. Note that “Bank 1” refers to the bank of cylinders on V-type engines that contains cylinder #1.

When diagnosing catalytic converter issues on any vehicle, it helps to understand what the term “catalytic converter” means. Put simply, it means that harmful constituents such as oxides of nitrogen, carbon monoxide, and several others are converted into innocuous substances like carbon dioxide and water vapor through contact with catalysts. In most modern catalytic converter designs, the catalysts are precious metals like platinum, iridium, rhodium, and others that react chemically with some exhaust gas constituents to convert these substances into other substances.

In terms of operation, a catalytic converter contains a porous core, or substrate that is coated with thin layers of catalytic metals. When exhaust gas flows through the pores in the core, the exhaust gas is exposed to the catalytic metals and provided the temperature of the core material has reached a temperature of around 1 400oF, the conversion of harmful substances starts and continues at the molecular level. Note that most modern catalytic converter designs include a dedicated electronically controlled heater element to reduce the core’s warm-up time to less than 20% of the time it takes to heat the core through contact with hot exhaust gas.

It is perhaps worth noting that catalysts in this or any other chemical reaction are not consumed in chemical conversion processes. Therefore, when an automotive catalytic converter’s efficiency decreases, it is not because the catalytic material has been consumed. Decreased efficiency results from factors and issues that either prevent the exhaust gas from flowing through the core, or more commonly, when contaminants like unburned fuel and oil vapors coat the catalysts, or as a result, begin to overwhelm the catalysts’ ability to convert some gaseous components in the exhaust stream to less harmful substances.

While modern catalytic converters can remove more than 95% oxides of nitrogen and similar percentages of carbon monoxide from the exhaust gas, the efficiency of catalytic converters cannot be monitored directly by a PCM. Therefore, all exhaust after-treatment systems use oxygen sensors, one upstream of the converter, and one downstream of the converter to make it possible for the PCM to infer a catalytic converter efficiency value. In practice, the upstream sensor measures the amount of oxygen in the exhaust stream, and based on this information, which switches rapidly between rich and lean, the PCM continuously adjusts the ratio of air to fuel in the air/ fuel mixture.

Because of this rapid switching, the signal voltage from the upstream oxygen sensor fluctuates rapidly, but in a fully functional system in which the catalytic converter has a high efficiency value, the signal voltage from the downstream oxygen sensor remains relatively constant at around the 450 mV mark. The PCM interprets the constancy of the downstream oxygen sensor’s signal voltage as a measure of the catalytic converter’s efficiency; any deviations from the signal voltages’ midpoint (450 mV) are interpreted by the PCM as reductions in the converter’s efficiency.

As a rule of thumb, PCM’s on all vehicles are programmed to set trouble codes when the catalytic converter’s efficiency drops below about 85%, although in some cases, the minimum efficiency threshold can be as high as 90%, which brings us to-

What causes code P0420 on Mitsubishi vehicles?

As with many other generic trouble codes, there is no single cause or set of circumstances that will cause code P0420 to set more frequently on Mitsubishi vehicles than on any other brand of vehicles.

When diagnosing this code, it helps to bear in mind the fact that while the technical implementation of some emission control strategies via effective control of the air/fuel metering systems through OBD II protocols sometimes differ between manufacturers, all OBD II systems are emission control systems first and foremost, as opposed to being diagnostic systems.

Therefore, almost any fault, failure, defect, or malfunction in the air/fuel metering systems of any modern Mitsubishi vehicle is likely to have the potential to affect a catalytic converters’ ability to clean up dirty exhaust gas. Put in another way, this means that the same faults, failures, defects, and malfunctions that affect the efficiency of a catalytic converter on a Mitsubishi vehicle will affect the efficiency of catalytic converters on any other brand of vehicle. See the section on Common Causes for more details on issues that can set code P0420 on Mitsubishi vehicles.

Where is the P0420 sensor located?

This image shows the location (arrowed) of the catalytic converter on a 2012 Mitsubishi Outlander, but note the red circle, which indicates a crack in the exhaust tube upstream of the converter.  This is a relatively common issue on some Outlander models, which can cause several different oxygen sensor related trouble codes to set.

On older Mitsubishi vehicles, the catalytic converter will be located in the exhaust system close to the engine to make maximum use of the hot exhaust gas to bring the converter up to operating temperature in the shortest possible time.

On later models, the catalytic converter is heated electrically, and it may therefore be placed further away from the engine. Note though that many catalytic converters strongly resemble mufflers and resonating boxes, so be sure to consult reliable service information for the affected application to locate and identify catalytic converters correctly.

What are the common causes of code P0420?

The possible causes of code P0420 on Mitsubishi vehicles are many and varied, and could include one or more of the following, but note that some possible causes could cause code P0420 to set as a secondary effect. These causes are marked with an [*], but note that additional codes relating to possible causes marked with an * may or may not be present in all cases-

  • Excessive oil consumption*- verify that oil consumption is within the manufacturer’s specifications
  • The use of unsuitable engine oil* – verify that the correct type and grade of oil is in the engine
  • Poor quality fuel* – verify that the fuel is clean, not contaminated, and of the correct octane rating
  • Worn or unsuitable sparkplugs*- verify that sparkplugs are in good condition and that sparkplugs of the correct type and heat grade are used
  • Engine oil contaminated with engine coolant – verify that engine oil is uncontaminated
  • Exhaust system leaks
  • Engine vacuum leaks
  • Clogged or dirty air filter element
  • The use of substandard aftermarket, or so-called “re-built” catalytic converters
  • Defects in the EGR system – see special note below
  • Defective oxygen sensors and associated wiring – see special note below
  • Defects and/or malfunctions in the catalytic converter’s heating system
  • Broken or damaged catalytic converter core/substrate
  • Failed or failing PCM, but note that since this is a rare event, the fault must be sought elsewhere before any control module is reprogrammed or replaced

SPECIAL NOTES: When diagnosing this code on Mitsubishi vehicles it is important to verify that no oxygen sensor and/or EGR (Exhaust Gas Recirculation) related codes are present since some oxygen sensor and EGR codes can either cause code P0420 to set, or contribute to its setting. Failure to verify that the oxygen sensors and EGR system is fully functional could result in a misdiagnosis, and quite possibly, the unnecessary replacement of a hugely expensive catalytic converter. END OF SPECIAL NOTES.

What are the symptoms of code P0420?

The most common symptoms of code P0420 on Mitsubishi vehicles are much the same across all Mitsubishi vehicles, and could include one or more of the following, but note that in some cases, there may be no symptoms present apart from a stored trouble code and illuminated warning light-

  • Stored trouble code and illuminated warning light
  • Additional codes may or may not be present in all instances of code P0420
  • Varying degrees of power loss may be present
  • The vehicle may be in a fail-safe or limp mode, but note that this will typically be the result of a failure in a system that had caused P0420 to set as a secondary effect. Put in another way, this means that code P0420 on its own will typically not initiate a fail-safe or limp mode
  • Fuel consumption may increase
  • Some readiness monitors may not initiate, or may not complete successfully
  • The vehicle will fail an emissions test

BAT Team Discussions for P0420

  • 2001 Honda Accord
    Error Code P0420 Cat eff. below threshold (Bank 1) 4 cyl. V-tec engine, 01 Accord with 220,000 miles. Should I replace the O2 Sensor in bank one, or is the Cat most likely shot ? And how would I know for sure? Can't pass inspection for emissions....
  • Dodge Intrepid code
    We need mileage and engine and year. Any other codes ? What is repair and tune up history? If it is the cat bad they are under federal warranty for 80k or 8 years. But one of the main causes of cat codes is bad tune up or engine miss or running rich. Also sometimes there is a pcm reflash for the pro...
  • 1999 Honda Accord coding P0420
    Well I've read numerous forums concerning my issue. Pre O2 sensor, cat, post O2 sensor and every combination in between. I have just over 65,000 miles on the car. Called American Honda and they told me my warranty was 8yr or 80K. Of course mine expired in Feb 07. After doing more research I found a ...
  • 01 Toyota Corolla with Cat eff code
    112,000 miles, auto, 1.8l. Has P0420 Cat eff code. Owner reports Cat was replace once under warranty. Graphed post Cat O2 sensor activity and saw it very active, close to same activity as Sensor 1. No other codes or problems found. Fuel trims look good. I'm going to send them to someone for new Cat...
  • 2005 Nissan Altima 2.5
    All, MIL came on on my '05 Altima after 102K miles. Mechanic found error code P0420. I was researching and found the following reference to a TSB" TSB #06004 -- MIL ON WITH DTC P0420: CATALYTIC CONVERTER IS UNDAMAGED. *TT (NHTSA ID #10019113, JANUARY 24 2006) I should note that I've noticed no...