P0721 code definition
The P0721 code is a sign that the TCM (transmission control module) isn’t properly converting incoming pulse signals.
What the P0721 code means
On vehicles with electronically-controlled transmissions, the OSS (output shaft speed sensor) is positioned at the end of the output shaft. It’s right before the drive axles so that it can take readings of the shaft’s speed. This is done primarily to determine when to shift, the schedule for torque converter engagement and EPC (electronic pressure control) inside the transmission.
If the PCM/ECM (powertrain control module/engine control module) has determined that the OSS sensor is malfunctioning, the P0721 code will be triggered. However, it may still be able to work by substituting values from other sensors to determine torque converter operation, shift timing, and EPC pressure.
What causes the P0721 code?
Causes of P0721 include:
- Problems with the electric wiring or connections
- A malfunctioning transmission fluid temperature sensor
- A malfunctioning output shaft speed sensor
- A malfunctioning speed sensor
What are the symptoms of the P0721 code?
The code is generally accompanied by the Check Engine Light. You may also notice that the speedometer isn’t reading correctly. Delayed shifting or trouble shifting at all are also very common symptoms. You may hear a grinding sound or experience resistance when trying to shift. In some cases, the engine will actually stall when you come to a stop. The engine could even misfire.
However, in some rare cases, there actually aren’t any real noticeable symptoms except for the P0721 code itself.
How does a mechanic diagnose the P0721 code?
Most mechanics will begin by removing the OSS to see if it’s the root of the problem. If any of its teeth are missing or have become worn, they’ll need to be replaced, but the code will most likely disappear after the OSS is repaired and reinstalled. They may also test the resistance put forth by the OSS.
The wiring harness should also be inspected for any indication of damage or loose connections. Dirty terminals would be enough to cause the code as well. In vehicles with TFT (transmission fluid temperature) sensors in-line with the OSS sensors, the former should also be checked for resistance.
Common mistakes when diagnosing the P0721 code
One big mistake is diagnosing the P0721 as a problem with engine/turbine speed or as an issue with the internal transmission. Another pretty common misdiagnosis is blaming fuel delivery problems. In any case, the mistake leads to unnecessarily replacing the vehicle speed sensor.
How serious is the P0721 code?
If your speedometer isn’t reading correctly, you’ll want to get the P0721 code addressed right away. Not knowing how fast you’re going could result in an accident or an otherwise avoidable speeding ticket.
Otherwise, the issue is mostly going to be an annoying inconvenience. No one likes stalling out or hearing a jarring noise when their vehicle is shifting (that noise also reflects internal damage – another reason to get this addressed ASAP).
What repairs can fix the P0721 code?
The work necessary to make the P0721 code disappear may include:
- Removing the OSS sensor and inspecting its condition, especially its teeth and the sensor’s resistance
- Inspecting the wiring harness for loose connections or wires that have been damaged
- Checking the power that goes into the OSS sensor as well as the PCM harness
Additional comments for consideration regarding the P0721 code
With some vehicles, the OSS sensor works with the TFT. This means that if there is a problem with the second sensor, the first will suffer as well. The P0721 code could also be caused by poor wiring between the two. If another code is present that has to do with the TFT, that’s a pretty good sign that it’s playing a part in the P0721 code.
Need help with a P0721 code?
If your vehicle is displaying a P0721 code, YourMechanic is here to help. We can send a certified mobile mechanic to your place of work or home – whichever is most convenient. Just sign up on our site or call 1-800-701-6230 to speak with one of our service advisors. They’ll walk you through the process and provide you with a free quote.