Mercury Mariner: P0720 → OSS Circuit → Malfunction


P0720 is a generic OBD II trouble code, meaning it has the same meaning for your Mercury Mariner as it does for any other vehicle. Indicates that your Mariner’s OSS (Output Shaft Speed ​​Sensor) is reporting a malfunction. It is usually caused by a faulty OSS, or a wiring problem.

The OSS works by turning a gear at the end of the transmission. As it rotates, the voltage across the OSS changes. This signal is sent to your Mariner’s ECM, which uses it to determine when and how hard to shift gears.

P0720 Fast DataP0720 Mercury Mariner

  • Does not affect engine operation
  • Usually caused by a faulty output shaft sensor
  • This can cause the transmission to shift abruptly

P0720 Symptoms: Mercury Mariner

You should notice symptoms when your Mariner has P0720. Since the code is related to the transmission, your engine should run fine. The concern will be how the transmission behaves without the OSS signal

These are the most common P0720 symptoms:

  • Erratic and delayed gear shifting
  • The speedometer and/or cruise control do not work
  • service engine light
  • Bad fuel consumption

In very rare cases, the vehicle may fail or even die when stopping.

Causes of Mercury Mariner P0720

These are the most common causes of P0720:

Mercury Mariner P0720 Diagnostic

Gear wear on the OSS

Your Mariner’s OSS works by turning gear and converting that movement into a signal that the ECM can use. Some makes and models use plastic gear. This gear can wear out over time. What was the speedometer like before the code appeared? Did he move a lot? Did it seem accurate? That’s a good indication that the OSS gears could be compromised. If it has been, you will need to replace the gear to fix the problem.

bad OSS

The output speed sensor itself may be bad. This can occur throughout the life of the vehicle. You will need a multimeter to diagnose it.

Here’s a great video on how to diagnose OSS with a multimeter.

Wiring problem

Wiring problems can cause P0720. The OSS wiring is lower to the ground than many cables. It also has a greater distance to travel to the ECM. With these two factors working against it, it is more common for the OSS to have wiring problems related to it than many other parts of the vehicle. An open/short or poor ground will trigger P0720

1. Check the harness connection

Look at where the wire harness connects to the OSS. Is it corrosion-free? Is it connected well? If not, clean it and replace it if necessary. Wire harness problems are very common when diagnosing this code.

2. Broken and damaged wiring

Check if any part of the wiring harness has been compromised. Look for wires that have been burned or frayed. Anywhere close to the leak is a good place to look.

Transmission fluid temperature sensor

In some vehicles, the OSS is connected to the TFT sensor. If there is a problem with the TFT sensor or the voltage coming from it, it will affect the OSS. This will launch P0720.

Conclusion: P0720

P0720 is relatively easy to diagnose. Although it can be a challenge to get to the OSS, Good luck repairing your Mercury Mariner. If there is anything you would like to add, leave a comment below.

See also: P0700


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