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

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By Reinier (Contact Me)
Last Updated 2021-10-20
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0420 Catalytic converter system, bank 1 -efficiency below threshold
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Catalytic converter, wiring, HO2S

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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 NOTES: Although generic OBD II trouble codes such as P0420 – “Catalytic converter system, Bank 1 – Efficiency Below Threshold”  can, and do affect all OBD II compliant vehicles, some vehicle makes and models are sometimes more susceptible to some generic trouble codes than most other vehicle makes and models. Under real-world driving and operating conditions, there are several reasons for some generic trouble codes to be more common on some vehicles than on others, but typical reasons include-

  • poor part/sensor/component design
  • poor routing of electrical wiring
  • poor placement of electrical components under seats, carpets, and center consoles
  • the exposed location of some parts/sensors/components on some vehicles
  • prolonged exposure to contaminants like road salt in some locations that cause damage to some parts and components on some vehicles, but not on others
  • difficulty in accessing some parts/sensors/components, thereby preventing or inhibiting the regular inspection, servicing, and/or replacement of affected parts/sensors/components during routine and/or scheduled servicing
  • Poor fuel quality in some locations

However, note that not all models of a particular vehicle make will necessarily be equally susceptible to one or more generic trouble codes, because car manufacturers often correct issues like poorly designed parts and/or poor routing of wiring at various points during a model’s production run. Nonetheless, this article will discuss generic trouble code P0420 – “Catalytic converter system, Bank 1 – Efficiency Below Threshold” as it applies to the BMW models that are most commonly affected by it. END OF SPECIAL NOTES.

OBD II fault code P0420 is a generic trouble code that is defined as, P0420 – “Catalytic converter system, Bank 1 – Efficiency Below Threshold”, and is set when the PCM (Powertrain Control Module) detects that the efficiency of the catalytic converter on 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. Also, note that on BMW vehicles, the PCM (Powertrain Control Module) is known as the DME (Digital Motor Electronics).

Current exhaust emissions regulations require that all vehicles be fitted with catalytic converters to convert harmful exhaust emissions into innocuous substances like carbon dioxide and water vapor. Moreover, current exhaust emission regulations also require that all OBD II compliant vehicles be fitted with devices and systems that can monitor the efficiency of all catalytic converters on the vehicles continuously.

As a result of these requirements, all vehicles are fitted with one or more catalytic converters, whose operation and efficiency is monitored by at least one oxygen sensor, and sometimes, by other sensors that include exhaust gas temperature sensors, exhaust backpressure sensors, and dedicated NOx sensors that collectively, represent a highly refined and sophisticated monitoring system.

However, in its simplest form, this monitoring system obtains input data from a single oxygen sensor that is located at the point where the exhaust gas exits the catalytic converter. Here is how this system works-

In their most basic form, a catalytic converter contains a porous substrate to a) allow exhaust gas to flow freely through the substrate, and b) to increase the surface area of the substrate as a means to bring as much of the substrate into contact with the exhaust gas. However, all the surfaces of the channels in the substrate are coated with metals like platinum, iridium, rhodium, and others, with these metals acting as catalysts* that initiate and maintain the chemical processes that convert some harmful exhaust products into less harmful substances.

* Catalysts are materials that initiate chemical actions/reactions without being consumed in the process- hence the term “catalytic” converter.

In terms of operating principles, however, the substrate (and its catalytic coatings) must be heated to about 7500F for the chemical process to begin and to become self-sustaining. In most modern vehicles, the catalytic converter is typically heated by an embedded electrical heating element that is controlled by the DME. This strategy reduces the warm-up time significantly, which in turn, reduces the amount of exhaust emissions vehicles generate while their catalytic converts are heating up.

When a catalytic converter reaches its optimal temperature, it begins the conversion of exhaust gas, but since the DME cannot monitor the conversion process directly, it depends on input data from the oxygen sensor that monitors the exiting exhaust gas to infer the efficiency of the converter. As a practical matter, and assuming that the catalytic converter is working properly, the composition of the exhaust gas that passes over this sensor will be relatively constant.

Put differently, this means that since the oxygen content of the exhaust gas creates an electrical current in the sensor, it follows that a constant oxygen level will be reflected in a relatively constant electrical current. However, a constant electrical signal does not necessarily mean that the catalytic converter is operating above a minimum allowable efficiency threshold.

Therefore, the DME compares the signal from the downstream oxygen sensor with the signal from the upstream oxygen sensor, whose electrical signal is expected to fluctuate between about 0.1V, to about 0.9V as a result of throttle inputs that change the composition of the air/fuel mixture. On most modern vehicles, a high signal output from the upstream sensor indicates a rich mixture, while a low signal output indicates a lean mixture. Therefore, a mean value of around 0.45V indicates an air/fuel mixture that is at, or close, to stoichiometric, which is an air/fuel mixture that contains one part of fuel for every 14.7 parts of air.

In a fully functional engine management system, the DME will adjust the air/fuel mixture up to about twenty times per second to correct both lean and rich conditions. We need not delve into the complexities of how this system works here, but suffice to say that collectively, these adjustments are known as Short-term Fuel Trims, and they serve as the principal reference the DME uses to gauge the operation of the downstream sensor, whose electrical signal should remain relatively constant at around 0.45V.

Thus, as a practical matter, the DME compares the output signals of the upstream and downstream oxygen sensors to infer the efficiency level of the catalytic converter. For instance, if no oxygen sensor-related fault codes are present, and the electrical signal of the downstream sensor follows the pattern of the upstream sensor, the DME draws the inference that the catalytic converter is not working as it should.

Conversely, and again assuming no oxygen sensor codes are present, but the signal from the downstream sensor remains relatively constant while the signal from the upstream sensor fluctuates between high and low, the DME draws the inference that the catalytic converter is working at a high efficiency rate.

However, since the proverbial devil lives in the detail, the DME is programmed to recognize when the signal from the downstream sensor begins to mirror the signal from the upstream sensor. For instance, the downstream sensor’s signal might begin to “spike” during large throttle inputs, which would mean that the catalytic converter is no longer able to cope with high hydrocarbon loads, which in turn, translates directly into a reduced efficiency rate.

On older vehicles, the minimum allowable efficiency rate can be as low as 75 percent, but on new or late-model vehicles, the minimum allowable threshold is typically between about 85 to 90 percent. Thus, when the DMR recognizes that the efficiency of the affected catalytic converter is approaching, or has reached the minimum allowable efficiency threshold, it will set code P0420, and illuminate a warning light as a result.

What causes code P0420 on BMW vehicles?

Many BMW models and especially BMW models with forced induction are known to consume huge quantities of oil- up to 1 quart per five hundred miles of driving and sometimes more.

While the manufacturer considers this oil consumption rate normal, its practical effect is to coat the catalytic metals in catalytic converters with a layer of unburnt or partially burnt oil, which prevents contact between the metals and the exhaust gas. As might be expected, this reduces the efficiency of catalytic converters on affected BMW models at hugely accelerated rates, and it is common to see complete catalytic converter failures on some BMW models occurring well within their mandated warranty periods.

Where is the P0420 sensor located?

This image shows the location of a catalytic converter on an E60 5-Series BMW model. Note that while it is possible to replace a catalytic converter on some BMW models on a DIY basis, this process usually involves removing heat shields, splash guards, and other parts to gain access to the exhaust system.

Therefore, the procedure can sometimes be challenging even for professional mechanics. Therefore we strongly recommend that you seek professional assistance with diagnosing and replacing catalytic converters on any BMW application.

What are the common causes of code P0420?

The most common causes of code P0420 on BMW applications are for the most part, largely similar and could include one or more of the following-

  • Excessive oil consumption
  • Mechanical damage or impacts that could damage the fragile substrate in the converter’s housing
  • Mechanical damage to the engine such as leaking engine valves, worn piston rings, or scuffed/worn cylinder walls that cause poor combustion
  • Worn, defective, or unsuitable sparkplugs
  • The use of a substandard aftermarket or a so-called “refurbished” catalytic converter
  • Long use of the original catalytic converter
  • The use of non-stock or unauthorized performance-oriented engine management software that can and often does create overwhelmingly large hydrocarbon loads
  • Misfires caused by leaking or otherwise defective fuel injectors
  • Excessive fuel pressure over extended periods

Diagnostic tip:  Note that while issues like-

  • defective oxygen sensors
  • damaged oxygen sensor wiring
  • exhaust leaks
  • exhaust manifold leaks
  • clogged mufflers
  • defective sensors

– that include exhaust temperature sensors, exhaust backpressure sensors, and even NOx sensors are often cited as possible causes of code P0420, the fact is that when these sensors are defective, they typically supply the DME with implausible, inaccurate, or invalid input data. Therefore, when other codes are present along with code P0420, the P0420 code is almost always the result of one or more of the additional codes, as opposed to P0420 being the cause of the additional codes.

What are the symptoms of code P0420?

The most common symptoms of code P0420 could include one or more of the following, but note that this code very rarely produces serious or noticeable drivability issues if no additional codes are present-

  • Stored trouble code and possibly an illuminated warning light
  • The strong odor of unburned fuel may be present, or in some cases, a strong odor of sulfur may be present
  • Fuel consumption may increase somewhat

NOTE: If additional codes are present along with P0420 and the vehicle is exhibiting serious drivability issues like severely or even noticeable power loss, poor idle quality, frequent and unpredictable stalling, serious misfires, engine overheating, or even a limp mode, the better option would be to have the affected vehicle inspected by a professional mechanic. In some cases, the drivability issues are caused by problems and defects that can only be diagnosed with BMW-specific diagnostic equipment, and P0420 might be the result of issues that cannot be diagnosed with generic equipment.

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