OBD-II trouble code
P0720: Output Speed Sensor Circuit Malfunction
The transmission's output speed sensor is reporting an erratic, missing, or impossible signal. This is the sensor the transmission control module uses to know how fast the output shaft is spinning — without it, the TCM can't shift correctly or know what gear to be in.
Quick facts
- System
- Powertrain
- Category
- Transmission / Speed Sensor
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $800
- DIY difficulty
- Intermediate DIY
What does P0720 mean?
P0720 sets when the transmission control module loses or doesn't trust the signal from the output speed sensor — the sensor mounted on the transmission case that reads the speed of the output shaft (the shaft that turns the driveshaft on RWD vehicles or the differential on FWD vehicles). The TCM uses this signal as ground truth for vehicle speed. It compares output speed to input speed (engine RPM filtered through the torque converter) to confirm the transmission is actually in the gear it's been commanded into. It uses output speed to decide when to upshift and downshift. It also reports speed to the speedometer on many platforms and to the cruise control system.
When the output speed signal goes erratic or drops out, the TCM loses the ability to know what's actually happening inside the transmission. Most TCMs respond by entering limp mode — locking into a single gear (often third) and disabling all adaptive shifting. You'll still be able to drive, but shifts disappear, the speedometer may stop working or read incorrectly, and the engine may behave oddly because the PCM also uses speed data for fuel and emissions decisions.
The causes split into three buckets. Sensor failure is the most common — output speed sensors are exposed to transmission fluid and heat, and they're a known wear item on most automatic transmissions past 100,000 miles. Wiring and connector problems make up the second bucket, especially on transmissions where the sensor sits low on the case and gets road debris or moisture exposure. The third bucket is internal — metal shavings in the transmission fluid building up on the sensor tip, or in rare cases physical damage to the sensor tone ring inside the transmission.
Common causes
- Failed output speed sensor (most common)
- Damaged or corroded sensor connector
- Broken or chafed wiring between sensor and TCM
- Metal debris from transmission wear coating the sensor tip
- Damaged sensor tone ring inside the transmission (rare)
- Low or contaminated transmission fluid affecting sensor operation
- TCM internal fault (rare — usually shows other codes too)
- Recent transmission repair where the sensor wasn't reseated properly
- Aftermarket gear or tire size change without recalibration on certain platforms
Symptoms
- Check engine light on with P0720 stored
- Speedometer reads zero or behaves erratically
- Transmission stuck in one gear (limp mode)
- Harsh or delayed shifts before limp mode engages
- Cruise control disabled or refusing to engage
- ABS or traction control may also throw codes due to lost speed reference
- Engine RPM doesn't correlate to road speed
Diagnostic steps
- 1.Read live data and compare commanded gear, input speed, and output speed on a brief test drive. A failed output speed sensor will show as zero or erratic output speed at any vehicle speed.
- 2.Check transmission fluid level and condition. Low or burnt fluid can affect sensor operation and points to needed transmission service.
- 3.Inspect the sensor connector visually. Corrosion, road damage, or a loose pin is a common fix.
- 4.Test sensor resistance against manufacturer spec — most output speed sensors read 200-1500 ohms depending on type.
- 5.Wiggle-test the wiring harness from the sensor to the TCM while watching live data. An intermittent signal during the test confirms wiring damage.
- 6.If sensor and wiring check out, the next step is dropping the pan to inspect for metal debris and tone ring damage — a much bigger repair.
Repair cost
$150 – $800
Output speed sensor replacement is typical at $150-$400 — the sensor is $40-$150, labor is $100-$250 depending on accessibility. Wiring repair: $100-$300. Transmission fluid service if needed: $150-$300. If the tone ring is damaged internally, you're looking at $1,500-$3,000+ because the transmission needs to come apart. Confirm the sensor is the actual cause before assuming the worst case.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with transmission fluid service preselected. Adjust labor rate and vehicle category to fit your situation.
Related repairs
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.