2018 Infiniti QX50 Diesel 3.0L Persistent P0720 Code After Speed Sensor Replacement

[zerouali med]

Hi all,

I’m having an issue with my 2018 Infiniti QX50 (3.0L diesel, not VC-Turbo, 7-speed auto). The car has lost speed on highways, and the transmission won’t shift into 4th, 5th, or 6th gears. In DS mode, I can manually shift to 1st, but it won’t go to 2nd or 3rd. The diagnostic scan shows a P0720 code (output speed sensor circuit). I replaced the output speed sensor with a new one baught from nissan canada , but the problem persists—no change in symptoms, and the same P0720 code still appears.

Has anyone dealt with this on a QX50/QX70 diesel or similar Infiniti/Nissan model? I suspect it could be a wiring issue, valve body, or TCM problem based on some research. The fluid looks okay but hasn’t been changed recently. Any advice on next steps, like checking the wiring harness, testing the sensor circuit, or whether this is likely a valve body failure? I’m based in Morocco, where there are no Infiniti dealerships, and local Nissan service centers have refused to assist with my issue. I’ve run diagnostics, but i have some fear that its not sure. Does anyone know of a reliable professional online diagnostic service that can help me troubleshoot this problem remotely? I’m looking for experts familiar with Infiniti/Nissan vehicles to guide me through advanced checks or interpret scan data effectively.

Thanks for any insights!

 

[Julian Antorelli]

Hi all,

I’m having an issue with my 2018 Infiniti QX50 (3.0L diesel, not VC-Turbo, 7-speed auto). The car has lost speed on highways, and the transmission won’t shift into 4th, 5th, or 6th gears. In DS mode, I can manually shift to 1st, but it won’t go to 2nd or 3rd. The diagnostic scan shows a P0720 code (output speed sensor circuit). I replaced the output speed sensor with a new one baught from nissan canada , but the problem persists—no change in symptoms, and the same P0720 code still appears.

Has anyone dealt with this on a QX50/QX70 diesel or similar Infiniti/Nissan model? I suspect it could be a wiring issue, valve body, or TCM problem based on some research. The fluid looks okay but hasn’t been changed recently. Any advice on next steps, like checking the wiring harness, testing the sensor circuit, or whether this is likely a valve body failure? I’m based in Morocco, where there are no Infiniti dealerships, and local Nissan service centers have refused to assist with my issue. I’ve run diagnostics, but i have some fear that its not sure. Does anyone know of a reliable professional online diagnostic service that can help me troubleshoot this problem remotely? I’m looking for experts familiar with Infiniti/Nissan vehicles to guide me through advanced checks or interpret scan data effectively.

Thanks for any insights!

Hello,

It sounds incredibly frustrating, especially with the limited local support. Let's break down what's happening and your next steps.

The P0720 code for the output speed sensor circuit is a critical starting point. Since you've already replaced the sensor with a genuine part from Nissan Canada and the code remains, it strongly suggests the issue isn't the sensor itself but rather the signal it's sending—or not sending—to the Transmission Control Module (TCM). This makes your suspicion about a wiring issue very likely.

Here's a logical troubleshooting path, starting with the most probable and easiest things to check:

1. Wiring Harness Inspection
This is the most common cause of a persistent speed sensor code after replacing the sensor. A damaged wire, a corroded connector pin, or a poor ground connection can all interrupt the signal.

Visual Check: Carefully inspect the wiring harness leading to the output speed sensor. Look for any signs of chafing, cuts, or damage, especially where the wires might rub against the transmission case, engine, or other components.

Connector Integrity: Unplug the connector and inspect the pins inside. Look for any bent, broken, or corroded pins. Make sure the pins are firmly seated and that the connector snaps on securely.

Check for a Good Ground: A poor ground connection can also cause an intermittent or absent signal. You'll want to check the ground points for the transmission and the TCM.

2. Testing the Circuit
If the visual inspection doesn't reveal anything, you'll need a multimeter to test the circuit itself.

Reference Voltage: With the ignition on, check the voltage at the sensor's connector. You should see a stable reference voltage (typically around 5V or 12V, depending on the system). If you're not getting a reading, there's a break in the power or ground wire.

Continuity: With the battery disconnected, use the multimeter's continuity function to check the wires from the sensor's connector back to the TCM connector. This will confirm if the wires are intact. You'll need a wiring diagram for your specific vehicle to know which pins to test.

3. Diagnosing Further
If the wiring checks out, you're looking at the next likely culprits:

TCM (Transmission Control Module): The TCM is the brain of the transmission. If it's not correctly interpreting the signal from the speed sensor, it will prevent the transmission from shifting. A faulty TCM could be the cause, but it's less common than a wiring issue.

Valve Body: While a valve body issue is possible, it usually manifests in a different way, such as delayed or harsh shifts, or getting stuck in a specific gear. The P0720 code specifically points to a speed sensor circuit problem, so you should exhaust all other possibilities before assuming a costly valve body replacement is necessary.

Regarding an Online Diagnostic Service:
Given your location, an online service is an excellent idea. Finding a specialist with deep knowledge of Infiniti/Nissan is key. Look for a service that can connect you with a professional who can interpret the "live data" from your scan tool. They can tell you what values to look for as you drive the car (if it can be driven safely) and can help you interpret the voltage readings when you do the circuit tests.

Some services offer professional-level remote diagnostics, where a technician can guide you step-by-step through checks and help you understand the data your scanner is showing. This could be the most effective way to get to the root of the problem without a local dealership.

I hope this helps you move forward with a logical plan. Start with the wiring harness inspection, as it's the most likely and least expensive fix. Good luck!

 

[zerouali med]

“Thanks so much for taking the time to break this down so clearly. Your step-by-step explanation really good to understand where to start, and I appreciate the logical approach. thank you again for your help!”

 

[Julian Antorelli]

“Thanks so much for taking the time to break this down so clearly. Your step-by-step explanation really good to understand where to start, and I appreciate the logical approach. thank you again for your help!”

Thank you for saying so! Let me know how you make out.