OBD2 Diagnostic Code
P0720
Output Speed Sensor Circuit Malfunction — How to Diagnose & Fix It
P0720 kills your speedometer, scrambles your transmission shifts, and can knock out your ABS — all from one small sensor. Before anyone quotes you a full transmission rebuild, read this. In most cases the actual fix is a bolt-on sensor swap that takes under an hour.
Severity
6 / 10
Moderate. Drivability and safety systems are affected. Do not delay beyond a few days.
Safe to Drive?
Conditionally
Short, slow trips only. Speedometer may be inaccurate and ABS can be partially disabled.
Typical DIY Fix Cost
Low – Moderate
Sensor replacement is usually straightforward. Wiring repair adds labor but rarely requires a specialist.
🔍 What This Code Means
Your transmission has two speed sensors: an input speed sensor that reads how fast the shaft entering the transmission is spinning, and an output speed sensor (OSS) that reads how fast the shaft leaving the transmission is spinning. The output shaft connects directly to your driveshaft and wheels — so its speed tells the computer exactly how fast the vehicle is actually moving.
P0720 means the PCM (Powertrain Control Module) is receiving an abnormal signal from the output speed sensor circuit — or no signal at all. The computer uses that reading to control shift timing, torque converter lockup, cruise control, ABS, stability control, and the speedometer. When the signal disappears, all of those systems lose their reference point at the same time.
💡 The output speed sensor is typically a magnetic sensor mounted on the outside of the transmission housing. It reads teeth on a reluctor ring attached to the output shaft. As each tooth passes, it generates a small voltage pulse — the faster the shaft spins, the more pulses per second. The PCM counts those pulses to calculate vehicle speed.
This code does not mean your transmission is failing internally. In the majority of verified cases the transmission itself is fine — the problem is the sensor, its wiring, or the tone ring it reads.
⚡ Common Symptoms
P0720 symptoms range from barely noticeable to immediately alarming depending on how completely the signal is lost. Some drivers only notice one symptom; others experience several simultaneously.
Speedometer reads zero or fluctuates
The most obvious sign. With no speed signal, the gauge drops to zero even while moving, or bounces erratically at highway speed.
Harsh or erratic transmission shifts
The transmission shifts based on vehicle speed. Without accurate data it may shift too early, too late, or slam between gears unpredictably.
Transmission stuck in one gear / limp mode
Many transmissions default to second or third gear when they lose speed sensor data — a fail-safe to get you home, not a sign of internal failure.
ABS warning light on
ABS compares wheel speed to vehicle speed. When vehicle speed is unknown, the ABS module flags a fault and disables the system.
Cruise control inoperative
Cruise control cannot hold a speed it cannot measure. Most systems disable entirely when P0720 is active.
Check engine light only
On some vehicles the failure is intermittent — the speedometer works most of the time and the only sign is the stored code.
🎯 Most Likely Causes
Listed from most common to least common. Always start at #1. The majority of P0720 cases are resolved before reaching cause #4.
1
Failed or failing output speed sensorThe sensor itself has worn out, shorted internally, or been damaged by heat and vibration. This is the root cause in close to half of all P0720 cases. The sensor is a bolt-on component, usually held by a single bolt, and replacement is the most straightforward repair on this code.
~ 45% of cases
2
Damaged or corroded wiring and connectorThe sensor harness runs along the transmission and is exposed to heat, road debris, and transmission fluid. Chafed insulation, a broken wire inside an intact jacket, or a corroded connector pin causes intermittent or complete signal loss. This is the cause in roughly 1 in 4 cases.
~ 25% of cases
3
Damaged reluctor ring (tone ring)The reluctor ring is the toothed wheel the sensor reads. A missing or broken tooth causes dropout spikes in the speed signal that the PCM interprets as a malfunction. On high-mileage vehicles this ring can crack or lose teeth without warning. Inspection requires removing the sensor first.
~ 15% of cases
4
Low or severely degraded transmission fluidContaminated or low fluid causes irregular pressure pulses and erratic clutch engagement that can create a noisy, inconsistent signal at the output shaft. Always check fluid level and color before replacing any parts — burnt fluid smells sharp and turns dark brown or black.
~ 10% of cases
5
PCM fault or software issueRarely, the module itself develops a fault in the circuit that reads the sensor. On some vehicles a TSB-covered software update resolves P0720 without any hardware replacement. This should only be considered after the sensor, wiring, and fluid have all been verified as good.
~ 5% of cases
🔧 What to Check First
Run through these three checks before spending any money. They take about 30 minutes and require only basic tools. They identify the cause in most P0720 cases.
1
Check transmission fluid level and conditionWith the engine warm and running (for most vehicles — confirm your model's procedure), pull the transmission dipstick. Healthy fluid is bright red and smells mildly sweet. If the fluid is dark brown, black, or smells burnt, that fluid needs to be changed before any sensor diagnosis is meaningful. Low fluid level is a separate problem that needs to be addressed alongside P0720 — never add fluid without finding out where it went.
2
Inspect the sensor connector and wiring harnessLocate the output speed sensor on the transmission housing — it typically has a single electrical connector and one mounting bolt. Unplug the connector and inspect both sides for green corrosion, bent pins, or pushed-back terminals. Trace the harness toward the firewall looking for chafed sections where the wire has rubbed against a sharp edge. Wiggle the harness while watching live data on a scanner — an intermittent drop in speed signal confirms a wiring problem without replacing anything.
3
Test the sensor with an inexpensive multimeterUnplug the sensor connector. Set your multimeter to AC voltage. Have a helper slowly roll the vehicle forward while you probe the sensor terminals. A healthy magnetic speed sensor generates a small AC voltage — typically 0.5V to 2.5V — that rises as speed increases. Zero volts at any speed means the sensor has failed. A reading that drops in and out confirms intermittent failure. This test takes five minutes and tells you definitively whether the sensor is the problem before you buy one.
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⚠️ Common Misdiagnosis
P0720 is a code that makes shops nervous because it involves the transmission — and nervous shops sometimes recommend expensive solutions before ruling out cheap ones. These are the three traps to watch for.
⚠ Misdiagnosis #1 — Transmission rebuild or replacement recommended for a limp-mode symptom
Limp mode feels alarming — the transmission is stuck in one gear and the car barely moves. But limp mode is a programmed fail-safe, not a sign of internal mechanical failure. A transmission stuck in second gear due to a missing speed signal is functioning exactly as designed. Any shop recommending a rebuild before testing the sensor and its wiring is skipping the diagnostic process entirely.
Potential savings: Substantial — sensor replacement vs. full rebuild
⚠ Misdiagnosis #2 — New sensor installed without inspecting the wiring or reluctor ring
Replacing the sensor clears the code temporarily in some cases — but if the actual cause is a broken wire or a damaged reluctor ring, P0720 returns within a few days of driving. The correct approach is to test the sensor in place first, then inspect the wiring and tone ring before ordering any parts. Swapping sensors blindly often means buying two sensors when one wiring repair would have fixed it.
Potential savings: Eliminates unnecessary repeat repairs
⚠ Misdiagnosis #3 — ABS module replacement recommended alongside P0720
Because P0720 knocks out the ABS warning light simultaneously, some shops flag the ABS module as a separate problem requiring separate repair. In almost every case the ABS fault is a downstream effect of the missing vehicle speed signal — not an independent failure. Fix the speed sensor first and retest. The ABS light will clear on its own once the module has valid speed data again.
Potential savings: ABS module repair vs. sensor replacement
💰 Estimated Repair Cost
DIY cost = parts only. Shop cost = parts plus labor. Ranges vary by vehicle, region, and shop rate. Use these as a planning guide — not a fixed quote.
Output speed sensor replacement
DIYVery Low
ShopLow–Moderate
LevelEasy
Wiring harness repair (chafed section)
DIYMinimal
ShopLow
LevelEasy
Connector replacement / pigtail repair
DIYVery Low
ShopLow
LevelModerate
Transmission fluid service
DIYLow
ShopModerate
LevelEasy–Moderate
Reluctor ring replacement
DIYModerate
ShopModerate–High
LevelAdvanced
PCM reprogramming / software update
DIYN/A
ShopModerate
LevelProfessional
✅ In most verified P0720 cases the total repair cost is very low when the diagnosis starts with the sensor and wiring — not with the transmission. A full transmission rebuild is rarely the answer to this code.
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🚗 Most Reported On
P0720 can appear on any vehicle with an automatic or CVT transmission. These models report it most frequently based on owner data and verified repair records.
Dodge Ram 1500 / 2500 (2003–2012, 45RFE / 545RFE)
Jeep Grand Cherokee (2005–2013, 5-spd automatic)
Chrysler 300 (2005–2010, 5-spd automatic)
Honda Accord (2003–2007, 5-spd automatic)
Nissan Altima (2002–2006, 4-spd automatic)
Ford F-150 (2004–2008, 4R70W / 4R75W)
Chevrolet Silverado (2000–2007, 4L60E / 4L80E)
Toyota Tacoma (2005–2015, A340F automatic)
❓ Frequently Asked Questions
Can I drive with P0720 active?
Only for short distances to reach a shop. With no reliable vehicle speed signal, your transmission cannot shift correctly, your speedometer may be inaccurate, and safety systems like ABS and stability control may be partially disabled. Avoid highway driving entirely until this is repaired.
Why does my speedometer read zero even though I'm moving?
The speedometer on most modern vehicles reads directly from the output speed sensor signal. When that signal is lost — due to a failed sensor, broken wiring, or a damaged reluctor ring — the instrument cluster receives no data and defaults to zero. Fixing P0720 restores the speedometer in the vast majority of cases.
What is the difference between P0720 and P0500?
P0720 is a transmission-side code that specifically targets the output speed sensor inside or on the transmission. P0500 is a broader Vehicle Speed Sensor malfunction that can originate from a sensor mounted on the transmission, differential, or wheel hub depending on the vehicle. They often have overlapping symptoms but point to different sensor locations. Always pull the freeze frame data to see which system flagged the fault first.
Can a low transmission fluid level cause P0720?
Indirectly, yes. Low or degraded fluid causes erratic clutch pack engagement, which creates inconsistent speed signals that can trigger P0720. Always check fluid level and condition first — it is the cheapest and fastest check available and rules out a common contributing factor before replacing any sensors.
How long does it take to replace the output speed sensor?
On most vehicles the output speed sensor is a bolt-on component accessible from outside the transmission. Replacement typically takes 30 to 60 minutes for a competent DIYer. Some vehicles require moving an exhaust shield or repositioning a wiring harness for access, which adds time but does not change the fundamental simplicity of the job.
🔗 Related Codes
These codes share similar root causes or commonly appear alongside P0720.
This guide is for informational purposes only. Always consult a qualified mechanic before performing any vehicle repair. OBDPlatform is not responsible for actions taken based on this content.