P0720
PowertrainOutput Speed Sensor Circuit Malfunction
The gearbox has a sensor that counts how fast the output shaft is spinning, and the ECU has decided that signal has gone missing or started reading nonsense. That speed reading feeds your speedometer, the gear change timing, and on a lot of cars the ABS and cruise control as well. For you that usually shows up as a jumpy or dead speedo and a gearbox that shifts oddly, and on many autos it will throw itself into limp mode to protect itself.
ⓘ Information only. This page provides general educational information about fault code P0720. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0720 mean?
P0720 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Output Speed Sensor Circuit Malfunction.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, and on some cars a transmission warning lamp too
- • Speedometer reading erratically, jumping about, or sitting at zero while you're moving
- • Harsh, delayed, or confused gear changes from an automatic box
- • Gearbox dropping into limp mode, often stuck in third gear, or refusing to shift cleanly
- • ABS or cruise control packing up at the same time, because they share the same speed data
- • Slightly worse economy when the box is shifting badly or sitting in limp mode
Possible causes
Causes commonly associated with P0720, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The output speed sensor itself has failed, the most common cause. They sit in a hot, dirty spot and the internals give up over time
- 2. Wiring to the sensor chafed, corroded, or broken. Heat and road salt do a number on the loom down there over the years
- 3. A loose or corroded connector at the sensor. Push it home, check for green crud on the pins before you blame the sensor
- 4. Low or filthy automatic transmission fluid. Old fluid and metal swarf clinging to the sensor tip will scramble the reading
- 5. A fault in the internal gearbox harness, which is a bigger and more expensive job to get at
- 6. A poor earth or a supply voltage problem feeding the sensor circuit
- 7. A failing transmission control module or ECU, uncommon, so leave this near the bottom of your list
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Plug in a scanner and read every stored code plus the freeze frame data from when P0720 logged. A P0715 or input speed sensor code alongside it points you somewhere different to P0720 on its own
- 2. Get the car up and have a proper look at the sensor, its connector, and the loom running to it. Most of these turn out to be a damaged connector or a chafed wire rather than a dead sensor
- 3. Check the gearbox fluid level and condition. Burnt smell, dark colour, or glitter in the fluid tells its own story and may need sorting first
- 4. Back-probe the connector and check you've got a clean supply voltage and a solid earth at the sensor
- 5. Test the sensor itself. A scope is ideal to watch the signal while the wheels turn, but a multimeter checking resistance against the workshop spec will catch an open or shorted unit
- 6. Clear the code and road test. Note whether it comes straight back or only shows up intermittently, which usually means a wiring or connector fault rather than the sensor
Common questions about P0720
Should I just buy a cheap sensor off eBay or stick with a genuine one? +
For the speed sensor a decent aftermarket part from a known brand is fine and saves you a fair bit over the dealer price. Where the cheap stuff catches people out is the bargain-bin no-name units, which can read slightly off and set the code straight back, leaving you fitting it twice. Spend a little more for a reputable brand. If you're on a German auto or something with a sensitive box, OEM is worth the extra for peace of mind.
Can I keep driving like this or do I need to stop? +
Short hops to get it looked at are usually fine, but I wouldn't ignore it. If the box has gone into limp mode you'll be stuck in one gear with no power, which is awkward on a motorway slip road. The bigger worry is a box that's shifting harshly because it can't see the output speed, since that puts real strain on the internals. Get it diagnosed soon rather than running it for weeks.
Is this going to fail the MOT? +
The code on its own isn't an MOT item, but a warning light glowing on the dash at the time of test can fail you under the lamp checks. There's also a knock-on risk: if the same sensor feeds your ABS, a lit ABS light is a straight fail. Sort the fault and let the lights go out over a few drive cycles before you book it in.
What sort of money am I looking at to fix it? +
If it's just the sensor, expect somewhere around £80 to £180 all in at a decent independent garage, parts and labour. A wiring repair can be cheaper or dearer depending on how buried the damaged section is. A fluid and filter service to clear up a contaminated-fluid cause runs roughly £120 to £250. Main dealers will charge noticeably more for the same work, and if it turns out to be the transmission control module you're into several hundred pounds, sometimes four figures with programming.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →