Technical Description of OBD-II Trouble Code
The circuit range/performance of the reductant tank temperature sensor
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to many OBD-II vehicles (1996-newer). This includes vehicles from Mercedes, Sprinter, Ford, GMC, Chevrolet, etc. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration.
A stored code P205B indicates that the powertrain control module (PCM) has detected an abnormal voltage level in the reductant tank temperature sensor circuit. This code is found exclusively in vehicles with a clean burning diesel engine.
The catalyst system is responsible for reducing almost all exhaust emissions, although certain applications also have a NOx trap.
Exhaust gas recirculation (EGR) systems go a step further in reducing NOx. However, larger and more powerful diesel engines today cannot meet strict federal (U.S.) emission standards with just the EGR, diesel particulate filter/catalytic converter, and NOx trap. This is why selective catalytic reduction (SCR) systems were invented.
SCR systems inject a reductant compound or diesel exhaust fluid (DEF) into the exhaust in front of the diesel particulate filter, NOx trap, and/or the catalytic converter using the reductant injection valve (solenoid). The precise timing of DEF injection increases the temperature of the filtration element, allowing it to function more efficiently. This improves the longevity of the filtration element and reduces harmful exhaust emissions released into the atmosphere. The entire SCR system is monitored and controlled by either the PCM or a stand-alone controller, which interacts with the PCM. The controller monitors the O2, NOx, and exhaust temperature sensors, among other inputs, to determine the appropriate time for DEF (reductant) injection. Precise DEF injection is crucial for maintaining exhaust temperature within acceptable parameters and optimizing pollutant filtration.
The reductant/regeneration supply pump pressurizes the DEF in the liquid reductant system for use when needed. The PCM monitors the supply pump voltage for consistent fluctuation and load percentage. The PCM also monitors one or more pressure sensors in the reductant supply system to detect any leaks.
If the PCM detects an abnormal voltage level in the reductant tank temperature sensor circuit that is outside acceptable parameters, it will store a code P205B and may illuminate the malfunction indicator lamp (MIL). Multiple ignition cycles with a failure may be required for MIL illumination.
Example of a DEF reductant tank:
What is the severity of this DTC?
A stored code P205B should be considered severe and addressed as quickly as possible. It could disable the SCR system, and catalyst damage may occur if the conditions that led to the code being stored are not rectified promptly.
What are some of the symptoms of the code?
Symptoms of a P205B trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P205B troubleshooting steps?
To diagnose a code P205B, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and access to vehicle-specific diagnostic information.
Refer to your vehicle information source to find a technical service bulletin (TSB) that matches the vehicle’s year, make, and model, as well as the engine size, stored codes, and exhibited symptoms. A TSB may provide helpful diagnostic information.
Use the scanner connected to the vehicle diagnostic connector to retrieve all stored codes and relevant freeze frame data. It is recommended to write down this information before clearing the codes and then test-drive the vehicle until the PCM enters readiness mode or the code is reset.
If the PCM enters readiness mode at this time, the code is intermittent and may be more challenging to diagnose. In such cases, the conditions that led to the code being stored may need to worsen before an accurate diagnosis can be made.
If the code is immediately reset, the next step in the diagnosis involves searching your vehicle information source for diagnostic flowcharts, connector pin-out charts, connector face views, and component testing procedures/specifications.
Step 1
Use the DVOM to test the reductant temperature sensor according to the manufacturer’s specifications. Any components that do not test within the maximum allowable parameters should be considered defective.
Step 2
If the actual reductant temperature is within specifications, the code P205B persists, and the sensor in question is functioning correctly, use the DVOM to test the input and output signal circuits between the temperature sensor and the PCM/SCR controller. Disconnect all controllers before conducting the DVOM tests.
