P20E8 – Reductant Pressure Too Low


By Reinier (Contact Me)
Last Updated 2020-12-19
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P20E8 Reductant Pressure Too Low -

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What Does Code P20E8 Mean?

OBD II fault code P20E8 is a generic trouble code that is defined as “Reductant Pressure Too Low” and is set when the PCM (Powertrain Control Module) detects an insufficient pressure in the reductant injection system between the injection pump and the injection nozzle on SCR catalytic converters.

NOTE: This code only applies to diesel vehicles that are fitted with SCR (Selective Catalytic Reduction) exhaust after-treatment systems.

While advances in diesel combustion technologies such as Exhaust Gas Recirculation have gone some way towards reducing the levels of harmful NOx (Oxides of Nitrogen) in diesel exhaust, diesel fuel still produces significantly more NOx than gasoline. Therefore, the latest iteration of exhaust emissions regulations includes specific requirements to further reduce the levels of NOx in diesel exhaust by at least 90% from previously allowed levels.

Because of this requirement, car manufacturers have developed an exhaust after-treatment system that targets NOx specifically in terms of converting it into less harmful substances such as water vapor, nitrogen, and trace amounts of carbon dioxide, as opposed to preventing its formation during the combustion process. This process is known as SCR (Selective Catalytic Reduction), and it occurs in an oxidizing environment in a purpose-designed catalytic converter through the addition of an oxidizing agent.

As a practical matter, the oxidizing agent is a mixture of water and urea known as ADBlue, or DEF (Diesel Exhaust Fluid), from which ammonia is extracted during the oxidizing process. However, for the process of converting NOx to harmless substances to be effective, the reductant (DEF), needs to be injected into the catalytic converter in precisely metered amounts, and at a very specific pressure to ensure an even distribution of the reductant throughout the catalytic converter’s substrate.

For instance, if too little reductant is injected due to insufficient injection pressure or some other defect, the conversion process may not initiate, or if it does, it may not complete successfully. On the other hand, if too much reductant is injected, the excess reductant may coat the surface of the substrate, which could also not only prevent the conversion process from initiating but could also effectively destroy the substrate.

Thus, to make the injection system work reliably, the system uses a computer-controlled injection pump to inject the metered reductant into the catalytic converter at a specific pressure, and under very specific operating conditions to ensure efficient distribution of the reductant.

In a fully functional system, both the temperature and level of reductant in a dedicated storage tank are monitored continuously. If any defect, malfunction, or failure occurs in the system that prevents the PCM from injecting reductant into the catalytic converter at a specified pressure, the PCM will recognize that it cannot control the conversion of NOx effectively, and it will set code P20E8 and illuminate a warning light as a result.

Where is the P20E8 sensor located?

This image shows a simplified schematic diagram of a typical Selective Catalytic Reduction diesel exhaust after-treatment system. Note that although the actual components of these systems vary greatly between applications, all such systems nevertheless follow this general layout and pattern.

Nonetheless, it is important not to mistake the catalytic converter for the DPF (Diesel Particulate Filter), which performs an entirely different function, although the two components often resemble each other very closely. Thus, to avoid confusion and misdiagnoses, bear in mind that the DPF is always located upstream of the catalytic converter.

What are the common causes of code P20E8?

Common causes of code P20E8 are largely similar across all applications, and could include one or more of the following-

  • Defective, malfunctioning, or failed injection pump
  • Contamination of DEF that affects the liquid’s density and/or purity- see note below
  • Low reductant fluid level
  • Defective, or malfunctioning reductant heater element
  • Leaks in the pressure feed line or injection nozzle
  • Programming errors in the reductant injection system’s dedicated control module
  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
  • 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

NOTE: DEF consists of a mixture of water and liquid urea in a ratio that ensures that both components of the mixture freeze and thaw at the same rate. Thus, if the mixture is contaminated with water or any other substance, one part of the mixture could conceivably remain frozen in sub-zero temperatures while the other thaws, which could produce issues with both injection rates and pressures. Note also that in cases where the reductant had become contaminated, resolving the problem definitively typically requires the replacement of the entire injection system.

What are the symptoms of code P20E8?

Symptoms of code P20E8 are similar across all applications, and could include one or more of the following-

  • Stored trouble code and illuminated warning light
  • Depending on both the application and the nature of the problem, multiple additional codes could be present along with P20E8
  • Catalytic converter failure could result if the problem is not resolved promptly
  • In some applications, the presence of code P20E8 can produce a no-start condition that will persist until the fault is found and corrected
  • In some cases, and depending on the nature of the problem, the PCM may initiate a fail-safe or limp mode that will persist until the fault is corrected, or until the vehicle is switched off. In the latter case, the no-start condition will persist until the problem is resolved

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