Technical Description of OBD-II Trouble Code
Malfunction in the Downshift Switch Circuit
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to various OBD-II vehicles (1996-newer). This includes vehicles from Ford, Acura, Vauxhall, Aston-Martin, Mercedes-Benz, Sprinter, and others. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
If your vehicle has stored a code P0816, it means that the powertrain control module (PCM) has detected a malfunction in the downshift switch circuit. This type of code is usually found in vehicles equipped with paddle shifters.
The transmission control module (TCM) may be a separate module or part of the PCM. Downshift (paddle) shifter buttons are typically located on the steering wheel horn pad, within easy reach of the driver’s fingers. They are used as a type of sport shifter, commonly found in high-performance models. The PCM monitors the transmission gear selection in comparison to the downshift switch input signals to ensure smooth operation. Additionally, the PCM monitors the voltage of the downshift switch circuit to ensure it falls within the specified parameters.
If the PCM detects that the transmission gear selection does not match the gear selection signal input from the downshift switch, or if the switch circuit voltage is outside the allowable range, a code P0816 may be stored and the malfunction indicator lamp (MIL) may illuminate. The MIL may require multiple ignition cycles with a failure for it to illuminate.
What is the severity of this DTC?
A stored code P0816 indicates that there is either a serious electrical issue or some type of mechanical failure. In any case, it is important to rectify the conditions that led to the storage of this code as quickly as possible.
What are some of the symptoms of the code?
Symptoms of a P0816 trouble code may include:
What are some of the common causes of the code?
The causes for this code may include:
What are some P0816 troubleshooting steps?
To diagnose a code P0816, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and access to vehicle-specific diagnostic information.
You can use your source of vehicle information to find a technical service bulletin (TSB) that matches the year, make, and model of your vehicle, as well as the stored codes and exhibited symptoms. If you find a relevant TSB, it may provide helpful diagnostic information.
Connect the scanner 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 either enters readiness mode or the code is reset.
If the PCM enters readiness mode at this point, the code is intermittent and may be more challenging to diagnose. In such cases, the conditions that led to the storage of the code may need to worsen before an accurate diagnosis can be made.
If the code is immediately reset, the next step in your diagnosis will involve consulting 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 voltage, ground, and signal output at the appropriate downshift switch. Typical downshift switch circuits consist of either battery voltage when activated or ground when deactivated.
Step 2
If the system circuits are functioning properly, use the DVOM to test the downshift switch circuits to the PCM or transmission. Disconnect all controllers before using the DVOM for testing.
Step 3
Use your source of vehicle diagnostic information and the DVOM to test the downshift switches as required. Replace switches that do not meet the system specifications.
