P209B Diagnostic Trouble Code: Reductant Injection Air Pressure Sensor “B” Circuit Range/Performance

Fault code P209B is called “Reductant Injection Air Pressure Sensor “B” Circuit Range/Performance” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P209B

Stored code P209B means that the Powertrain Control Module (PCM) has detected an operating range mismatch in the reducer temperature sensor circuit. This code is only displayed on vehicles with diesel engines.

P209B Diagnostic Trouble Code: Reductant Injection Air Pressure Sensor "B" Circuit Range/Performance

Exhaust gas recirculation (EGR) systems are designed to reduce NOx in exhaust gases. But today’s high-powered diesel engines cannot meet strict emissions standards with EGR alone.

Therefore, particulate filters, catalytic converters and NOx traps are also used to deactivate pollutants. Selective catalytic reduction (SCR) systems were invented for this reason.

SCR systems inject a reducing compound into the exhaust before the particulate filter or catalytic converter. Precisely timed injection increases the temperature of the filter element and allows it to work more efficiently.

This makes the filter element last longer and allows less harmful exhaust gases to be released into the atmosphere.

The entire SCS system is monitored and controlled either by the PCM or by a separate controller that communicates with the PCM. The controller monitors the O₂, NOx, and exhaust temperature sensors to determine the appropriate time for reducing agent injection.

The reducing agent feed pump is used to pressurize the DEF system when necessary. The PCM monitors the supply pump voltage for constant fluctuations and load percentage.

The PCM also monitors one or more pressure sensors in the restorer supply system to determine if there is a leak in the system.

If the PCM detects a performance range mismatch in the air pressure sensor “B” circuit of the reducer injection system, or out of tolerance. Then code P209B will be stored and the Malfunction Indicator Light (MIL) may illuminate.

Symptoms of vehicle malfunctions

The main signal that an error P209B has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. The “Check engine” control lamp on the control panel will light up (the code will be stored as a fault).
  2. Other related fault codes may also be present.
  3. Reduced engine power.
  4. Black smoke coming out of the exhaust pipe.
  5. Increased fuel consumption.
  6. The engine may go into emergency operation.

Stored fault code P209B should be considered serious and corrected as soon as possible. The SCR system may be disabled, causing damage to the particulate filter and catalytic converter.

Factors that can cause this error code

The error code P209B can mean that one or more of the following problems have occurred:

  • The air pressure sensor of the reducer is defective.
  • Open or short circuit in the air pressure sensor circuit of the reducer injection system.
  • Exhaust gas leakage.
  • Problem with the restorer control module.
  • Bad electrical connection in the restorer control module circuit.
  • In rare cases, PCM malfunction or programming error.

How to fix or reset OBD-2 code P209B

Some suggested steps for troubleshooting and fix the error code P209B:

  1. Read all stored data and error codes with an OBD-II scan tool. To find out when and under what circumstances error P209B appeared.
  2. Clear the error codes from the computer memory and test-drive the vehicle to see if the fault appears again.
  3. If the error code appears again, visually inspect the electrical wires and connectors.
  4. Visually inspect and test the air pressure sensor of the restorer.
  5. If necessary, replace the restorer air pressure sensor and any defective wires.
  6. Check the restorer control module.
  7. If there are no defects, check the PCM module and its software part.

Diagnose and repair of problems

Before you begin the process of troubleshooting P209B, you should research the technical service bulletins (TSB) for your particular vehicle. In some cases, this can save you a lot of time by pointing you in the right direction.

The diagnosis itself should begin with a visual inspection of the restorer system harnesses and connectors. Burned or damaged wiring and connectors should be repaired or replaced before proceeding with testing.

Using a multimeter, test the power supply to the SCR control system. Check the fuses when the circuit is loaded to avoid misdiagnosis.

Use a multimeter to test the air pressure sensor of the restorer according to the manufacturer’s specifications. Components that do not test within the maximum allowable parameters should be considered defective.

If the restorer injection pressure is within specifications, code P209B is retained and the sensor in question is in working order. Use a multimeter to check the input and output circuits between the temperature sensor and the PCM / SCR controller.

Restorer injector sensor codes are often associated with feed pumps that have internal leaks.

On which vehicles does this problem occur most frequently

Fault code P209B can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • BMW
  • Dodge
  • Mercedes-Benz (Sprinter)

Fault code P209B can sometimes be found with other errors. The most common are the following: P209A, P209C, P209D, P209E.

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