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P0720 causes both transmission and speedometer issues simultaneously because the output speed sensor feeds both the TCM and instrument cluster. A failed output sensor often causes an erratic or dead speedometer alongside harsh/no shifting. The sensor is usually accessible on the outside of the transmission tail housing. See top-rated scanners on Amazon ↗
🗺️ Where Is the Problem?
Blueprint view - P0720 output speed sensor fault at transmission tail housing (reads output shaft/wheel speed)
These are statistical causes across ALL vehicles - your exact car may rank differently
For example, on a Honda 4-cyl the downstream O2 sensor causes P0720 64% of the time, but on a GM 5.3L V8 the catalytic converter is the cause 71% of the time. Get a probability ranking built specifically for your year, make, model, and mileage.
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🎯 Top Causes & Probability
55%
#1 - Most Likely
Failed Output Speed Sensor
The output speed sensor has failed internally. This sensor measures the rotational speed of the transmission output shaft, providing vehicle speed data to both the TCM and the speedometer. A completely failed sensor will cause an erratic or dead speedometer alongside shifting problems. The sensor is typically mounted at the tail housing of the transmission and is easily accessible.
🔩 Part
$20–$80
👨🔧 Labor
$50–$150
⚡ DIY Difficulty
Easy
20%
#2 - Check First
Debris on Sensor or Reluctor
Metal debris accumulates on the magnetic sensor tip and attenuates the signal. Additionally, the reluctor ring (tone wheel) at the output shaft can corrode or accumulate debris between teeth, reducing signal accuracy. Remove the sensor, clean the tip, and inspect the reluctor ring for damage or heavy corrosion before replacement.
🔩 Part
$0 (cleaning)
👨🔧 Labor
$0
⚡ DIY Difficulty
Easy
15%
#3 - Less Common
Wiring Fault - Output Sensor
A damaged, chafed, or corroded wire in the output sensor circuit causes signal loss. The output sensor connector is exposed to road debris, moisture, and heat. Inspect the connector and harness from the sensor to the TCM for damage. A wiggle test while monitoring sensor data on a scan tool quickly identifies intermittent wiring faults.
🔩 Part
$5–$50
👨🔧 Labor
$50–$200
⚡ DIY Difficulty
Medium
10%
#4 -
Differential Ring Gear Damage
On transfer cases and differentials where the output sensor reads directly off the ring gear, damaged or worn ring gear teeth produce an erratic signal. This is less common but more expensive - requires differential disassembly to inspect and repair. Suspect this if the code persists after sensor and wiring replacement.
🔩 Part
$200–$600
👨🔧 Labor
$300–$800
⚡ DIY Difficulty
Hard
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Check Speedometer Behavior - P0720 typically causes erratic or dead speedometer. If the speedometer reads correctly, the output sensor signal is reaching the instrument cluster - the fault may be in the TCM wiring specifically. If the speedometer is also dead, the sensor itself is likely failed.
- Remove and Clean Sensor - Locate the output speed sensor on the transmission tail housing. Remove and clean the magnetic tip. Inspect the tone ring through the sensor bore for missing/damaged teeth. Reinstall and retest before replacing.
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- Ask if they charge a diagnostic fee and whether it applies toward the repair
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- Check Google reviews for recent mentions of the specific repair you need