P20EE – SCR NOx Catalyst Efficiency Below Threshold Bank 1

Reinier

By Reinier (Contact Me)
Last Updated 2018-01-25
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P20EE SCR NOx Catalyst Efficiency Below Threshold Bank 1 -

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What Does Code P20EE Mean?

OBD II fault code P20EE is a generic code that is defined as “SCR NOx Catalyst Efficiency Below Threshold Bank 1”, and is set when the PCM (Powertrain Control Module) detects that the catalytic converter on Bank 1 is operating below an acceptable or allowable minimum efficiency threshold. Note that code P20EE only applies to applications that use SCR (Selective Catalytic Reduction) technology and that “Bank 1” refers to the bank of cylinders that contains cylinder #1.

NOTE: It should be noted that the presence of code P20EE rarely indicates a failure of a catalytic converter itself, and that other codes relating to the reductant injection system are almost invariably also present when code P20EE is present.  

CSR technology refers to a system in which precisely measured quantities of a liquid reductant (typically urea obtained from ammonia, and commonly known as “Diesel Exhaust Fluid”) is mixed in with the exhaust stream before the exhaust stream passes through a catalytic converter.

While non-CSR catalytic converters are fairly efficient and remain so for extended periods of time unless serious issues with excessive oil consumption or over fuelling arise, the addition of a reductant to a CSR compatible catalytic converter greatly enhances that converter’s ability to reduce especially NOx (oxides of nitrogen) in particularly diesel exhaust to harmless substances.

However, the downside to CSR technology is that the amounts of reductant must be added in precisely measured quantities and at specific times to be effective. For instance, if too much reductant is injected, or if reductant is injected at the wrong time, the reductant can crystallize and clog the matrix over which the catalytic metals are distributed, which greatly reduces the converter’s efficiency. On the other hand, if too little reductant is injected the available reductant does not cover the entire catalytic matrix, which also greatly reduces the converter’s efficiency.

In a fully functional CSR system, the PCM monitors the efficiency of the catalytic converter by comparing the voltage pattern of the #2 (downstream of the converter) oxygen sensor with that of the #1 (upstream oxygen sensor). In practice, the voltage pattern of the #1 sensor should fluctuate rapidly, while the voltage pattern of the #2 sensor should remain fairly constant if the converter is operating at a high efficiency level. Note that on some applications, the efficiency of the (SCR) catalytic conversion process is monitored by two exhaust gas temperature sensors, one upstream of the converter, and one downstream of the converter.

Thus, if the PCM detects that the voltage patterns of the two oxygen sensors are too similar, or that the exhaust gas temperature is too high on the converter’s exit side (based on pre-programmed values) it will recognize that the catalytic converter is operating below a minimum allowable efficiency threshold. When this happens, the PCM will inject a precisely calculated amount of reductant into the exhaust stream to correct the efficiency problem. Note that the amount of reductant that is injected is based on average fuel consumption as measured over the period since the last injection event.

If the injection of reductant does not cause in increase in the efficiency of the catalytic converter, as measured by both the oxygen and other sensors such as NOx, exhaust gas temperature sensor, and exhaust pressure sensors, the PCM will recognize that the affected catalytic converter is operating below an allowable efficiency threshold and it will set code P20EE and illuminate a warning light as a result. Note that some applications will also enter a failsafe or limp mode that will persist until the fault is corrected.

Where is the P20EE sensor located?

The image above shows a simplified schematic of a typical SCR reductant injection system. Note that while some design specifics vary between applications, all reductant injection systems consist of the parts, components, and circuits shown here. However, while the arrangement and actual location of parts, components, and circuits in the reductant injection system vary between applications, the reductant injection nozzle is always located upstream of the catalytic converter as shown here.

Also, note that since code P20EE can be caused by failures, malfunctions, and defects in almost any part of the reductant system, it is critically important always to refer to the manual for the affected application to locate and identify parts and components correctly. Failure to do this will almost certainly result in wasted time, misdiagnoses, the unnecessary replacement of expensive parts and components, and quite possibly additional damage not only to the reductant injection system, but also to the application’s electrical system.

What are the common causes of code P20EE?

NOTE: As stated elsewhere, additional codes will almost always accompany code P20EE, any one or more of which codes will more often than not be the root cause of P20EE. Therefore, it is important to record all fault codes and freeze frame data present before attempting to diagnose P20EE. Moreover, all additional codes must be resolved in the order in which they were stored before a diagnosis of P20EE is attempted, since doing so will often resolve code P20EE as well.

Nonetheless, some common causes of code P20EE could include the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors almost anywhere in the reductant injection system
  • Defective reductant injection pump
  • Defective reductant injection dosing module
  • Defective reductant injection nozzle
  • Defective sensors, including engine coolant sensor(s), exhaust gas temperature sensor(s), NOx sensor(s), exhaust pressure sensor(s), and others, depending on the application
  • Exhaust leaks upstream of the catalytic converter
  • Unauthorized or illegal modifications to the exhaust system, or engine/fuel management systems
  • Low reductant levels, or defective reductant level sensor (where fitted)
  • Dirty or contaminated reductant. Note that if reductant is contaminated, the entire reductant injection system may need to be replaced
  • Fluid leaks in the reductant injection system
  • Defective reductant heating system
  • Defective SCR catalytic converter
  • Failed or failing PCM or other control module. Note that these are rare events, and the fault must therefore be sought elsewhere before any control module is replaced

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