Diagnosing and Replacing the 2014-2018 INFINITI QX80 Transmission Speed Sensor
If your QX80 is shifting erratically or the check engine light is on, a transmission speed sensor could be the cause, but the fix may be more complex than you think.
- A P0720 code on a QX80 often points to a failed Transmission Control Module (TCM)/valve body, not just the sensor.
- This is a complex, high-labor repair requiring removal of the driveshaft and transmission tail housing; it is not a DIY job.
- Due to the high labor cost, use only top-tier aftermarket brands like Hitachi or NGK/NTK to avoid premature failure.
- Always get a professional diagnosis before buying parts to confirm the true source of the failure.
Is a Bad Transmission Speed Sensor Really the Problem?
For the 2014-2018 INFINITI QX80, a check engine light with code P0720 (Output Speed Sensor Circuit) is very common. While the code points to the speed sensor, the actual point of failure is frequently 🎬 Watch this video to see how the P0720 code is fixed. the Transmission Control Module (TCM) itself. In these transmissions, the speed sensors and TCM are often integrated into a single unit called the valve body. Due to heat, the internal connections on the TCM circuit board can fail, triggering the speed sensor code. Because of this common failure pattern, many professional shops and even INFINITI service bulletins recommend replacing the entire valve body assembly, not just an individual sensor.
This is Not a Simple DIY Job
The output speed sensor on the QX80 is located inside the transmission's tail housing. Replacing it requires draining the fluid, removing the driveshaft, the transmission support crossmember, the transmission oil pan, and the rear tail housing of the transmission. This is a complex and labor-intensive job that should be left to a qualified transmission shop. It is not recommended for most DIY mechanics.
Symptoms of a Failing Transmission Speed Sensor
When the TCM loses the signal from a speed sensor, you will likely notice significant issues with drivability. The most common signs include:
🎬 See what these shifting problems and RPM surges look like.- Harsh or Erratic Shifting: The transmission may slam into gear, shift at the wrong times, or hesitate between shifts.
- Check Engine Light: A failing sensor will almost always trigger a check engine light. Codes P0720 (Output Speed Sensor) and P0717 (Input Speed Sensor) are the most common.
- Stuck in Gear (Limp Mode): The transmission may go into a fail-safe or "limp" mode, where it gets stuck in a single gear (often 3rd) to prevent further damage.
- Inaccurate Speedometer: The speedometer may stop working or show an incorrect speed.
- Cruise Control Not Working: The cruise control system relies on speed sensor data and will not function if the signal is lost.
Choosing the Right Aftermarket Brand
If you and your mechanic decide to replace only the sensor, or if you are replacing the entire valve body with an aftermarket unit, brand quality is critical. For complex electronics, avoid cheap, unproven brands.
Tier 1: OEM or Equal
- Hitachi: Hitachi is a Tier 1 Original Equipment (OE) supplier for many Japanese automakers, including INFINITI. Choosing Hitachi is often the same as buying the genuine part without the dealership packaging and price.
- NGK/NTK: NTK (a brand of NGK) is a world-renowned sensor specialist. They engineer their parts to meet or exceed OEM standards and are known for their reliability and precision.
Both Hitachi and NTK are excellent choices that you can trust to restore proper function. Given the high labor cost involved in this repair, it is not worth saving a few dollars on a lower-quality part that may fail prematurely.
Cost of Replacing the Transmission Speed Sensor on a 2014-2018 INFINITI QX80
| Part / Service | Estimated Cost |
|---|---|
| Shop Labor | $450 - $750 |
| New Aftermarket Speed Sensor | $95 - $282 |
| New OEM Valve Body Assembly | $1,600 - $2,100 |
| Remanufactured Valve Body Assembly | $650 - $1,000 |
| Total Job (Sensor Only) | $545 - $1,032 |
| Total Job (Reman. Valve Body) | $1,100 - $1,750 |
Frequently Asked Questions
Can I replace the QX80 transmission speed sensor myself?
This is not recommended for a DIY mechanic. The output speed sensor is located internally, and accessing it requires removing the driveshaft and part of the 🎬 Watch a step-by-step teardown of this complex 7AT transmission. transmission housing. This job requires specialized tools and expertise.
Is it safe to drive with a bad transmission speed sensor?
It is not recommended. The vehicle may shift unpredictably, get stuck in gear, or have a non-functional speedometer, all of which are safety risks. Driving with these symptoms could also potentially cause more damage to the transmission.
Why can't I just replace the sensor instead of the whole valve body?
While the trouble code may point to the sensor, the root cause is often a failure within the Transmission Control Module (TCM), which is part of the valve body assembly. Replacing only the sensor may not fix the problem, leading you to pay for the expensive labor twice. A full diagnosis is key.
Are there any recalls for this issue?
As of now, there are no active safety recalls from INFINITI or the NHTSA for the transmission speed sensor or valve body on the 2014-2018 QX80. However, there are multiple Technical Service Bulletins (TSBs) related to transmission shifting issues that may require software updates or other repairs.
Helpful Videos
Technical Specifications
OEM Part Numbers: 31935-1XJ0D 31935-97X00 31935-1XJ0A 31705-76X5E (Valve Body) 31705-26X1A (Valve Body) 31705-76X8C (Valve Body)
This guide pertains to the input and output speed sensors for the RE7R01A 7-speed automatic transmission used in the 2014-2018 INFINITI QX80.
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
- INFINITI QX80:
- 🎬 Helpful Videos
- 🛍️ Shop This Part
- Is a Bad Transmission Speed Sensor Really the Problem?
- This is Not a Simple DIY Job
- Symptoms of a Failing Transmission Speed Sensor
- Choosing the Right Aftermarket Brand
- Cost of Replacing the Transmission Speed Sensor on a 2014-2018 INFINITI QX80
- Frequently Asked Questions
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