P1778 is a common Nissan trouble code related to the CVT transmissions that Nissan often uses in their vehicles. This particular code indicates that there are issues with the step motor.
This article is written specifically for vehicles manufactured by Nissan.
P1778 is almost always caused by a bad step motor itself, and rarely anything else.
P1778 Quick Facts
- The step motor adjusts the “step” by turning coils on and off after being commanded by the Serena’s TCM.
- P1778 is a mechanical code and does not indicate that the circuit signal from the step motor is bad.
- The code is thrown when the changing of the speed mechanism does not operate properly.
- You can replace the step motor without replacing the transmission.
What is a Step Motor, Anyway?
CVT transmissions use a pulley system to create any gear ratio that they want. This allows your Serena’s engine to cruise at the best RPM for power or efficiency, depending on what the driver needs at the time.
The step motor is responsible for taking the signal from the transmission control module and adjusting the pulley. The CVT transmission will usually stay in a lower gear ratio (and throw P1778) when the step motor is not working right.
There are almost always noticeable symptoms related to code P1778. Here are the most common ones:
- Lack of acceleration
- Check engine light
- Transmission stuck in “low ratio mode”
Serena P1778 Causes + Diagnosis
Here is how to go about diagnosing P1778 at home. We will start with the easiest and most obvious things first, and work our way to the things that are difficult to deal with at home.
Check the CVT Fluid
While it’s not the most common cause of P1778, you should always start by checking the CVT transmission fluid in your Serena. It’s easy to check, and if it’s really dirty or low, maybe you’ll get lucky.
Check The Electrical Connections
You should verify that the step motor wiring harness is not damaged in any way. While P1778 is not a circuit/wiring-related code (meaning, the signal coming from the wiring harness is within normal operating range), it is still worth taking a look at any portion of the harness that you can easily access.
Bad CVT or Step Motor
Unfortunately, the number one fix for P1778 is to replace the step motor. Getting to it means opening the transmission, while it is doable, it’s a tough job.
P1778 in the Nissan Serena is most likely caused by a bad step motor. They are replaceable and pretty dang cheap. While a transmission, on the other hand, is very expensive.
If you want to have a go at replacing it, check this video. This young Shadetree mechanic replaces the step motor fast and clears P1778: