P0420 – Catalyst System Efficiency Below Threshold (Bank 1) (GM)

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By Reinier (Contact Me)
Last Updated 2021-01-27
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0420 Catalyst System Efficiency Below Threshold (Bank 1)
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Catalytic converter, wiring, HO2S 2

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Table of Contents

  1. What Does Code P0420 Mean?
  2. Where is the P0420 sensor located?
  3. What are the common causes of code P0420?
  4. What are the symptoms of code P0420?
  5. Get Help with P0420

What Does Code P0420 Mean?

Special note on trouble code P0420 and GM 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 GM 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 of exhaust gas  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 innocuous 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 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 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, and any downward 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 GM 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 GM 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 GM 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 GM vehicle will affect the efficiency of catalytic converters on any other brand of vehicle. See the section on Common Causes for more details on specific issues that can set code P0420 on GM vehicles.

Where is the P0420 sensor located?

This image shows the location and placement (circled) of the two catalytic converters on late-model Corvette. Note that while the converters on many GM applications are located in similar positions upstream of mufflers and other exhaust components, in many other cases, the catalytic converter and exhaust manifold may be integrated into single unit. In these cases, the exhaust manifold and catalytic converter must be replaced as an assembly, since neither part can be purchased separately.

What are the common causes of code P0420?

The possible causes of code P0420 on GM vehicles are many and varied, and could include one or more of the following, but note that some possible causes could cause 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 or recommended 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 GM 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 GM vehicles are much the same across all GM 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 an 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

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