P0720 – Output Speed Sensor Circuit Fault (What It Means and How to Fix It)

You’re driving along…

Gear changes feel off.

Speedo might drop to zero.

Then the tranny starts acting strange.

That’s when P0720 shows up.

This one points to a problem with the transmission output speed sensor. The car loses track of how fast the transmission is actually spinning on the way out to the wheels.

What the output speed sensor does

The output speed sensor monitors how fast the transmission output shaft is turning.

In plain terms, it tells the ECU and transmission control module how fast the car is moving from the tranny side.

That information is used for:

• Shift timing
• Torque converter lockup
• Speedometer readings on some vehicles
• Comparing input vs output speeds to detect slip

If that signal disappears or doesn’t make sense, the tranny is basically guessing.

And guessing leads to rough shifts or limp mode.

What causes P0720

This is often a straightforward fault.

Failed output speed sensor
Internal failure or contamination on the sensor tip

Wiring damage
Heat, road debris, or corrosion taking out the signal wire

Loose or corroded connector
Very common, especially low down on the transmission

Reluctor ring damage
If the tone ring inside the transmission is damaged or dirty, the signal gets distorted

Transmission control module issues
Rare, but possible

Low or dirty transmission fluid
Can affect internal operation and sensor readings indirectly

What you’ll notice

This one usually shows up through tranny behaviour.

• Check engine light on
• Harsh or delayed gear shifts
• Tranny stuck in one gear
• Speedometer not working or dropping out
• Poor acceleration
• Tranny going into limp mode

Drivers often say “it feels like it’s stuck in gear” or “it bangs into gears.”

How to diagnose it

Scan for codes and check live data. Look at output speed readings. If it’s flatlined or erratic, you’re on the right track.

Compare input speed vs output speed. If one is reading and the other isn’t, that narrows it down fast.

Inspect the sensor and wiring. These sensors sit low on the transmission and take abuse from road grime and water.

Check the connector. Any corrosion or loose pins can kill the signal.

If everything external checks out, you may be looking at an internal issue like a damaged tone ring.

Can you still drive with P0720

You can, but it’s not a great idea.

If the tranny goes into limp mode, you’ll be stuck in one gear. That’s manageable short term, but not safe for longer trips or faster roads.

Continued driving can also cause unnecessary wear inside the transmission.

Best to get it sorted early.

How to fix it

• Replace the output speed sensor if it’s faulty
• Repair or replace damaged wiring
• Clean or replace corroded connectors
• Check and correct transmission fluid level and condition
• Inspect for internal damage if the fault persists

Most output speed sensors are relatively easy to access from outside the transmission.

No major strip-down required in most cases.

FAQs

Is P0720 the same as a vehicle speed sensor fault?
Sometimes they overlap, depending on the system. Many cars use the output speed sensor as the main vehicle speed input.

Will this cause hard shifting?
Yes. The gearbox relies heavily on this signal. Without it, shifts become harsh or unpredictable.

Can low transmission fluid cause this code?
Not directly, but poor fluid condition can affect transmission behaviour and confuse diagnostics.

Do I need to replace the whole gearbox?
Very unlikely. Most of the time, it’s a sensor or wiring issue.