P0816Generic code

P0816 Code: Downshift Switch Circuit

The P0816 code means there is an electrical fault in the downshift switch circuit. The main effect is that the manual mode of the automatic transmission will not shift down. The transmission keeps working in automatic mode.

Severity
Low
Can I keep driving?
Yes, no immediate risk
Estimated cost
$20 to $250
Difficulty
Medium
System
Transmission
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0816 code means

In the manual modes of an automatic transmission, the downshift switch sends the command to the computer.

It is a switch: the computer sees a high or low voltage depending on its position. A circuit fault, the P0816 code, can come from the switch, the wiring or the power supply.

If the signal fails, the manual mode will not let you shift down. The transmission keeps working in automatic mode.

Always start with the simplest items: fuse, relay and connector. Then check power, ground and signal with the component connected (using back-probes), and finally the continuity of each wire up to the computer. Only if all of that is good should you replace the component.

P0816 code symptoms

  • Check engine light on.
  • Manual mode of the transmission that will not shift down.

Likely causes of the P0816 code

  1. Damaged or corroded connector. The most frequent cause.
  2. Wiring cut or shorted between the switch and the computer.
  3. Failed switch, open or shorted internally.
  4. Missing power reference. Check whether other sensors are also failing.
  5. Computer. Uncommon; rule out everything above first.

How to diagnose the P0816 code step by step

  1. Read the switch value on the scan tool and compare it with what is expected.
  2. Inspect the connector for corrosion, water or loose pins.
  3. Check power and ground with the key on: a correct reference and less than 0.1 V of drop on the ground.
  4. Measure the signal with the connector plugged in and see whether it changes with the condition.
  5. Do the wiggle test on the harness with the scan tool in graphing mode.
  6. Check the continuity of each wire up to the computer (under 1 ohm).
  7. Watch the signal on the scan tool while you pull the paddle or move the lever.

How to fix the P0816 code

  • Repair the connector or the wiring.
  • Replace the switch if the tests confirm it.
  • Correct the real condition, if there is one.
  • Clear the codes and verify with a road test.

Technical reference data

  • The switch or sensor is powered from the computer: check the manual for the values.
  • These are approximate values: check the manufacturer data for your vehicle.

Brand-specific notes

The letter assignment and the location of the component change from one manufacturer to another, so check the manual for your vehicle.

Frequently asked questions about P0816

What does the P0816 code mean?
It is a generic OBD2 code that means there is an electrical fault in the downshift switch circuit. The cause can be the switch, its wiring or its power supply.
Can I drive with a P0816 code?
Yes, with caution. The transmission keeps shifting in automatic mode, but the manual mode will not shift down.
Does the switch have to be replaced?
Only if the tests confirm it. The connector and the wiring fail just as often.
What does the downshift switch do?
In the manual modes of an automatic transmission, the downshift switch sends the command to the computer.
How much does it cost to fix P0816?
As a rough guide, between $20 and $250.
Will a P0816 code clear itself?
It can go out if the connection was only loose or damp for a moment. A damaged connector, wire or switch will bring the P0816 code back.
What is the most common cause of P0816?
A damaged or corroded connector. Check it and the wiring before you buy a switch.

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