P0420 - Catalytic System Efficiency Below Threshold (Bank 1) (GM) - TroubleCodes.net (2023)

P0420 - Catalytic System Efficiency Below Threshold (Bank 1) (GM) - TroubleCodes.net (1)

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Last update 2021-01-27
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Codeerror locationprobable cause
P0420Catalyst System Efficiency Below Threshold (Bank 1)
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Catalyst, wiring, heated oxygen sensor 2

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  1. What does the P0420 code mean?
  2. Where is the P0420 sensor located?
  3. What are the most common causes of the P0420 code?
  4. What are the symptoms of the P0420 code?
  5. Get help with P0420

What does the P0420 code mean?

Special Note About 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 brands. This article therefore addresses the P0420 code as it is specific to GM vehicles.

OBD II trouble code P0420 is a generic trouble code defined as “Catalytic Converter System, Bank 1 - Efficiency Below Threshold” and is set when the PCM (Powertrain Control Module) detects a condition where the efficiency of the serving catalyst bank 1 has dropped below a minimum allowable threshold. Note that "bank 1" refers to the cylinder bank of V engines that contains cylinder #1.

When diagnosing catalytic converter problems on any vehicle, it is helpful to understand what the term "catalytic converter" means. Simply put, this means that the harmful components of the exhaust gases such as nitrogen oxides, carbon monoxide and much more are converted into harmless substances such as carbon dioxide and water vapor through contact with catalytic converters. In most modern catalytic converter designs, the catalysts are precious metals such as platinum, iridium, rhodium and others that chemically react with some components of the exhaust gases to convert these substances into harmless substances.

Operationally, a catalytic converter contains a porous core or substrate coated with thin layers of catalytic metals. When the exhaust gas flows through the pores in the core, the gas is exposed to the catalytic metals and since the temperature of the core material has reached a temperature of about 1400ÖF, pollutant conversion begins and continues at the molecular level. Note that most modern catalytic converter designs incorporate a special electronically controlled heating element to reduce core heating time to less than 20% of the time required to heat the core through contact with the 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 the efficiency of an automotive catalytic converter decreases, it is not because the catalytic material has been consumed. Reduced efficiency results from factors and issues preventing exhaust gases from flowing through the core, or more commonly when contaminants such as unburned fuel and oil vapors begin to overwhelm the catalyst's ability to convert some gaseous components in the exhaust stream into lesser pollutants.

Although modern catalytic converters can remove over 95% of nitrogen oxides and similar percentages of carbon monoxide from exhaust gases, catalytic converter efficiency cannot be directly monitored by a PCM. Therefore, all exhaust aftertreatment systems use oxygen sensors, one before the catalyst and one after the catalyst, to allow the PCM to infer a catalyst efficiency value. In practice, the upstream sensor measures the amount of oxygen in the exhaust stream and based on this information, which rapidly alternates between rich and lean, the PCM continually adjusts the air/fuel ratio in the air/fuel mixture.

This rapid switching causes the upstream HO2S signal voltage to fluctuate rapidly, but in a fully operational system where the catalyst is highly efficient, the downstream HO2S signal voltage remains relatively constant around the 450 mV mark. The PCM interprets the constancy of the signal voltage from the downstream lambda probe as a measure of the efficiency of the catalytic converter. Deviations from the midpoint of the signal voltages (450 mV) downwards are interpreted by the PCM as a reduction in the catalytic converter efficiency.

Typically, the PCMs in all vehicles are programmed to set trouble codes when the efficiency of the catalytic converter falls below 85%, although in some cases the lower efficiency threshold can reach 90%, leading us to the following:

What causes the P0420 code on GM vehicles?

As with many other generic trouble codes, there is no single cause or set of circumstances that causes the P0420 code to be set on GM vehicles more often than any other make of vehicle.

When diagnosing this code, it is helpful to consider the fact that the technical implementation of some emission control strategies by effectively controlling air/fuel metering systems through OBD-II protocols sometimes differs from manufacturer to manufacturer, all OBD-II systems are emissions control systems first, in contrast to diagnostic systems.

As such, it is likely that nearly any fault, failure, defect or malfunction in the air/fuel metering systems of a modern GM vehicle can impair a catalytic converter's ability to clean dirty exhaust gases. In other words, the same failures, failures, malfunctions and malfunctions that affect the efficiency of a catalytic converter in a GM vehicle will affect the efficiency of catalytic converters in every other make of vehicle. See the Common Causes section for more information on specific issues that can set the P0420 code on GM vehicles.

Where is the P0420 sensor located?

This image shows the location and placement (circled) of the two late model Corvette catalytic converters. Note that while in many GM applications the catalytic converters are in similar locations upstream of mufflers and other exhaust components, in many other cases the catalytic converter and exhaust manifold can be integrated into a single unit. In these cases, the exhaust manifold and catalytic converter must be replaced together as both parts are not available separately.

What are the most common causes of the P0420 code?

Possible causes of the P0420 code on GM vehicles are varied and may include one or more of the following, but note that a few possible causes may result in the P0420 being defined as an adverse event. These causes are marked with a [*], but note that additional codes related to possible causes marked with a "*" may or may not be present in all cases.

  • Excessive oil consumption* - Check that oil consumption is within manufacturer specifications
  • Using the wrong engine oil* - check that the engine contains the correct type and grade of oil
  • Poor quality fuel* - Check that the fuel is clean, not contaminated and has the correct or recommended octane rating
  • Worn or bad spark plugs* - Check that the spark plugs are in good condition and that they are the correct type and heat rating
  • Engine oil contaminated with engine coolant - check that engine oil is not contaminated
  • Leaks in the exhaust system
  • Engine vacuum leaks
  • Clogged or dirty air filter element
  • The use of inferior catalytic converters or so-called “reconditioned” catalytic converters
  • EGR system malfunctions - see special note below
  • Defective oxygen sensors and associated wiring - see special note below
  • Catalyst heater defects and/or malfunctions
  • Cracked or damaged catalyst core/substrate
  • Faulty or faulty PCM, but note that this is a rare event that needs to be troubleshooted elsewhere before a control module is reprogrammed or replaced

SPECIAL NOTES: When diagnosing this code on GM vehicles, it is important to verify that there are no oxygen sensor and/or EGR (exhaust gas recirculation) sensor codes as some oxygen sensors and EGR codes can cause the code to appear P0420 is set or contributes to its setting. Failure to verify that the oxygen sensors and EGR system are fully functional can lead to misdiagnosis and potentially an unnecessary replacement of an extremely expensive catalytic converter. END OF SPECIAL NOTES.

What are the symptoms of the P0420 code?

The most common symptoms of the P0420 code on GM vehicles are similar on all GM vehicles and may include one or more of the following, but note that in some cases there may be no symptoms other than a stored trouble code and an illuminated warning light-

  • Stored error code and illuminated warning light
  • Additional codes may or may not be present with all instances of the P0420 code
  • There can be varying degrees of energy loss.
  • The vehicle may be in a failsafe or squishy mode, but note that this is usually the result of a fault in a system that caused the P0420 to set as a side effect. In other words, the P0420 code alone will not typically initiate a failsafe or clear mode.
  • Fuel consumption can increase
  • Some readiness monitors may not start or may not complete successfully
  • The vehicle will fail an emissions test

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