What Does It Mean?
This diagnostic trouble code (DTC) is a generic powertrain code and applies to many OBD-II vehicles (1996-newer). This may include, but is not limited to, vehicles from Ford, Acura, Vauxhall, Aston-Martin, Mercedes-Benz, Sprinter, etc. Although generic, the exact repair steps may vary depending on the year, make, model, and powertrain configuration.
If your vehicle has stored a P0816 code, it means that the Powertrain Control Module (PCM) has detected a malfunction in the downshift switch circuit. This type of code is typically limited to vehicles equipped with paddle shifters.
The Transmission Control Module (TCM) may be a standalone module or part of the PCM. The downshift shift buttons (paddles) are usually located on the steering wheel horn pad near the driver’s finger position. They are used as a type of sport shifter, typically in high-performance models. The PCM monitors the transmission gear selection against the input signals from the downshift switch to ensure continuity. Additionally, the PCM monitors the voltage of the downshift switch circuit to ensure it falls within established 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 switching circuit voltage is not within the allowed parameters, a P0816 code may be stored and a Malfunction Indicator Lamp (MIL) illuminated. Several ignition cycles (with a failure) may be required for the MIL to light up.
How Severe Is This DTC?
A stored P0816 code indicates that a serious electrical issue or a type of mechanical failure has occurred. In any case, the conditions that contributed to the storage of a code of this nature should be rectified as soon as possible.
What Are Some of the Symptoms of the Code?
Symptoms of a P0816 fault code may include:
Manual downshift function (paddle) disabled
Gear shift indicator reflects an incorrect gear
No symptoms may appear
What Are Some Common Causes of the Code?
Causes
of this code may include:
Faulty downshift switch
Short or open circuits in the downshift switch circuit
Faulty PCM or programming error
What Are the P0816 Troubleshooting Steps?
A diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of vehicle-specific diagnostic information will be needed to diagnose a P0816 code.
You can use your vehicle information source to locate a Technical Service Bulletin (TSB) that matches the vehicle’s year, make, and model; as well as engine size, stored codes, and presented symptoms. If you find one, it may provide useful diagnostic information.
Use the scanner (connected to the vehicle’s diagnostic connector) to retrieve all stored codes and relevant freeze frame data. It is a good idea to record this information before clearing the codes, then test the vehicle until the PCM enters readiness mode or the code resets.
If the PCM enters readiness mode at that point, the code is intermittent and may be much harder to diagnose. If this is the case, the conditions that contributed to the code storage may need to worsen before an accurate diagnosis can be made.
If the code immediately resets, the next step in your diagnosis will require you to consult your vehicle information source for diagnostic flowcharts, connector pinout diagrams, 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 switching circuits consist of battery voltage (when activated) or ground (when deactivated).
Step 2
If the system circuits are functional, use the DVOM to test the downshift switching circuits to the PCM or transmission. Disconnect all controllers before using the DVOM for testing.
Step 3
Use your vehicle diagnostic information source and the DVOM to test the downshift switches as needed. Replace switches that do not test within system specifications.
Downshift switching codes are most often attributed to a switch failure