Error Code P20D9: What It Means & What To Do?

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A stored a code P20D9 in your diesel powered vehicle means that the powertrain control module (PCM) has detected insufficient voltage in the fuel supply control circuit for the exhaust aftertreatment system.

OBD code P20D9 means Exhaust Aftertreatment Fuel Supply Control Circuit Low Issue.

The Exhaust Aftertreatment Fuel Injector is located on the right rear cylinder head. Fuel is supplied to the Exhaust Aftertreatment Fuel Injector from the low pressure side of the high pressure fuel pump. An outlet tube is connected between the Exhaust Aftertreatment Fuel Injector outlet and the exhaust down pipe. When the engine control module (ECM) commands the injector ON fuel is sprayed directly into the exhaust system. The fuel ignites in the exhaust and produces the heat required for the exhaust aftertreatment regeneration.

What causes this problem with the Exhaust Aftertreatment Fuel Supply Control Circuit Low?

• Loose, broken, burned, or corroded wiring and/or terminals and connectors
• Defective exhaust after treatment fuel injector
• Defective exhaust gas temperature sensor
• Presence of an aftermarket off road exhaust system that causes a decrease in back pressure
• Severe exhaust leak upstream from the exhaust temperature sensor

1. Engine Control Module - The ECM plays a crucial role in newer vehicles, where many (if not all) of the essential functions of the car are controlled by the ECM. When the ECM has any issues, it can cause all sorts of problems with the vehicle, and in some cases even render it undriveable. Thus, treat it at the earliest.
2. Exhaust Gas Recirculation Valve – One of the most common symptoms of a problem with the vehicle’s EGR valve is a rough idle. It is not uncommon for EGR valves to malfunction and become stuck in the open position. This can lead to exhaust gas recirculation, which causes rough idling even when the conditions are not desirable. It may also contribute to a smell of fuel as well. Replace it immediately.
3. Diesel Exhaust Fluid Pump – The DEF Pump creates hydraulic pressure to transfer the DEF from the tank to the injector. If DEF pump fails to operate properly, code P20D9 will set. Thus, replace it now!
4. Powertrain Control Module – Engine performance issues are another symptom of a possible problem with the PCM. If the PCM has any issues, it may throw off the timing and fuel settings of the engine, which can negatively affect performance. A faulty PCM may cause the vehicle to experience a reduction in fuel efficiency, power, and acceleration. Repair it at the earliest.
5. Automatic Transmission Module – Is your car showing a delay in acceleration when you press the pedal? Is it not picking up the speed the way it used to? It could be one of the transmission module symptoms to warn you that the device is not working properly. Thus repair or replace it now!
6. Fuel Injector – If you suspect that an injector is clogged or malfunctioning, an injector balance test can isolate the bad injector. Scan tools that can disable injectors can isolate an injector for diagnostics. Engine rpm drop may not be an effective diagnostic method when performing a cylinder balance test where an injector is disabled. Thus, check for the fault and replace it now!
7. Diagnostic Tool – Find wide range of diagnostic tools to detect and correct the OBD code error.

If you are facing OBD error code P20D9, then there are significant common symptoms that can be felt or observed. We have worked out a list here to help you out in understanding this problem better.

Common Symptoms

• Excessive black smoke upon acceleration
• Illuminated service engine soon lamp
• A stored trouble code.

It is very important to fix the fault code P20D9. So here are some ways with which you can correct this issue:

• Powertrain Control Module repair/replace/reprogram
• EAS Fuel Injection Control System replacement
• Damaged wirings replacement

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Here are some steps which you should follow in order to diagnose this fault code:

• Exhaust after treatment fuel injector systems are used exclusively in diesel applications. Exhaust after treatment fuel injector systems use either water, a chemical agent, air, or a combination of the three to reduce exhaust emissions in automotive applications by reducing exhaust temperature and/or exhaust pressure in the diesel particulate filter or catalytic converter assembly.
• The exhaust after treatment fuel injector is typically mounted directly in the diesel particulate filter or catalytic converter and it is normally controlled electronically using a dedicated exhaust after treatment fuel injector controller that is used in conjunction with the PCM.
• If the PCM detects an abnormally high exhaust temperature, pressure, and/or noxious gas concentration, the exhaust after treatment fuel injector is activated and the reductant compound is sprayed into the diesel particulate filter or catalytic converter.
• The reductant compound in turn helps to reduce the exhaust gas temperature and/or pressure and noxious gases. The exhaust gas temperature sensor is typically a 2-wire sensor located in the exhaust down pipe. The exhaust gas temperature sensor is used in diesel engines, gasoline burning engines, and even turbocharged engines. In diesel engines, the sensor will likely be located near the diesel particulate filter section of the exhaust system.
• Turbocharged applications may place the exhaust gas temperature sensor in the exhaust input pipe, near the turbocharger. The exhaust gas temperature sensor is of the temperature reactive resistor variety. The sensor is supplied with a low voltage (usually 5-volts) power wire and a ground wire.
• As exhaust gas temperature increases, the level of resistance in the sensor decreases, allowing the feedback voltage signal to the PCM to increase. As exhaust temperature decreases (with the engine off), circuit resistance increases and the feedback voltage signal to the PCM decreases.
• The PCM recognizes these voltage variations as exhaust gas temperature and it reacts accordingly, adjusting ignition timing or the air/fuel ratio in order to lower the exhaust gas temperature and protect the catalytic converter. When the PCM demands the reductant injector "ON" it begins to monitor the upstream and downstream exhaust gas temperature, exhaust gas pressure, or heated oxygen sensor/s to determine if there is sufficient reduction temperature, pressure, or noxious gas levels.
• If the downstream levels fail to display significant reduction, a code will be stored and a service engine soon lamp may be illuminated. Several tools will be instrumental in successfully diagnosing this code. A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in performing a successful diagnosis.
• An infrared thermometer with a laser pointer will also be helpful. Begin with a visual inspection of all wiring and connectors. Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary.
• Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data.
• This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored. Continue by clearing the code and operating the vehicle to see if it returns. This will help to determine whether or not the malfunction is intermittent.
• After the codes are cleared, test drive the vehicle to see if the code returns. If the code fails to immediately return, you may have an intermittent condition. Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made.
• Continue your diagnosis with a visual inspection of the exhaust gas temperature/pressure (or heated oxygen) sensors and system circuitry. Pay close attention to areas where wiring and connectors are located near hot exhaust components. If all of the exhaust gas temperature/pressure (or heated oxygen) sensor circuitry appears to be in proper working order, then proceed with the diagnostic procedure.
• If certain items appear to be damaged, burned, or corroded, repair or replace them as required, then reset the code and retest the system. If no burned, damaged, or shorted wiring or loose connectors are detected, disconnect the exhaust gas temperature/pressure (or heated oxygen) sensor connector and remove the exhaust gas temperature sensor.
• Using the digital volt/ohmmeter, check the resistance of the exhaust gas temp/pressure (or heated oxygen) sensor. Typical resistance for this sensor should be approximately 150 ohms but double check manufacturer's specifications prior to testing.
• Normally, if there is less than 50 ohms of resistance, then you would replace the sensor.
• If the initial resistance is within specifications, leave the leads of the digital volt/ohmmeter connected and heat the resistor with a heat gun.
• Carefully observe the resistance level of the sensor (on the screen of the digital volt/ohmmeter) as the temperature is elevated to see if it decreases smoothly.
• If the resistance level does not decrease with heat and increase as it cools, replace the sensor. If everything has checked out thus far, turn the key to the "ON" position and test for the presence of a voltage (and a ground signal) at the exhaust gas temperature/pressure (or heated oxygen) sensor electrical connector.
• If there is no low voltage (usually 5-volts) on the signal wire, disconnect the PCM connector and perform a continuity test on the wire using the digital volt/ohmmeter. If there is continuity, the PCM may be faulty (PCM failure is rare and, if replaced, it will need to be reprogrammed).
• If no ground is present locate the source and check for continuity using the digital volt/ohmmeter. Repair or replace open or shorted wiring and connectors as required. If there is no voltage present on the sensor connector and all system circuitry is functioning normally, suspect a faulty reductant injector air controller.
• Controller failure is rare and will likely require reprogramming. Installing high performance exhaust systems (with no catalytic converters) can also cause this code to be set due to the lack of exhaust system back pressure.
• This condition can be rectified by installing an in line resistor (2.5 ohms) between the signal and ground wires and leaving the exhaust gas temperature sensor disconnected. The PCM will read the sensor resistance and pass it. Check local, state, and federal laws before removing emission related components.

Common Mistakes When Diagnosing the P20D9 Code

Most of the time we tend to diagnose the problem without completely identifying the real cause of it. It is important to follow all the diagnostic steps and examine each component carefully before correcting. Some of the components to be focused on are listed below;

• Fuel pressure regulator failure
• DEF pump failure
• Fuel injector failure
• Powertrain Control Module (PCM) failure
• Wiring/harness issue