P1778 – CVT Step Motor Circuit Malfunction (Nissan)

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By Reinier (Contact Me)
Last Updated 2023-11-21
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P1778 CVT step motor – mechanical malfunction
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Table of Contents

  1. What Does Code P1778 Mean?
  2. Where is the P1778 sensor located?
  3. What are the common causes of code P1778?
  4. What are the symptoms of code P1778?
  5. Get Help with P1778

What Does Code P1778 Mean?

OBD II fault code P1778 is a manufacturer-specific trouble code that is defined by carmaker Nissan as P1778 – “Step Motor Function”, or sometimes as P1778 – “Step Motor Circuit Malfunction”, and is set when the PCM (Powertrain Control Module) detects a defect or malfunction in the electrical circuits that control the step motor in the CVT transmission. Note that the abbreviation CVT stands for “Continuously Variable Transmission”.

Unlike conventional automatic transmissions that have a fixed number of gear ratios, CVT transmissions can cycle through an endless range of effective “gear ratios” between what would be first gear and top gear in a conventional transmission.

In terms of operating principles, CVT transmissions essentially consist of two adjustable and split pulleys and a drive belt that transfers power from one pulley to the other. To effect a “gear change” the two halves of one pulley, which is driven by the engine, are moved apart by pressurized transmission fluid. This action reduces the effective diameter of the pulley at the point where the drive belt runs between the two halves.

At the same time, the two halves of the second pulley, which is connected to the differential, are pushed closer together thereby  increasing the pulley’s effective diameter at the point where the drive belt touches the two halves of the second pulley.

In a CVT transmission, “first gear” would be established when the engine-driven pulley is set to its smallest effective diameter, and the effective diameter of the pulley that is attached to the differential is set to its biggest effective diameter. Thus, by increasing the effective diameter of the engine-driven pulley but at the same time, reducing the effective diameter of the second pulley by the same amount, the changes in the effective diameters of the two pulleys establish different ratios that are somewhat analogous to different gear ratios in conventional automatic transmissions.

In modern CVT transmissions, the movement of the two halves of each pulley is controlled by a step motor, also known as a “stepper motor” which rotates only a set number of degrees each time it is activated; simply by changing its polarity, the step motor can rotate in reverse by the same number of degrees. Moreover, the step motor controls a flow control solenoid, aka “ratio control valve” that regulates the flow of pressurized fluid to each pulley in equal amounts but in a cross-wise fashion.

This means that if the engine-driven pulley’s effective diameter increases, the effective diameter of the pulley that drives the differential decreases by the same amount, and vice versa. This ensures that the belt tension remains constant regardless of the changes in the effective diameters of the two pulleys.

As a practical matter, the combined effect of the operating principles of CVT transmissions and their programming means that the engine always runs at its most economical speed, regardless of what “gear” the transmission is in at any given moment. While this produces measurable fuel savings compared to similar vehicles with conventional transmissions, CVT transmissions are generally unreliable and expensive to repair when they fail, which effectively negates the fuel savings.

The most common failures on Nissan’s CVT transmissions involve failures of the step motor, with failures of the ratio control valve following closely behind. Arguably, the third most common failure involves issues in the step motor’s wiring, with poor and/or corroded connections implicated in most such failures. In the case of code P1778, which refers specifically to wiring issues in the step motor’s circuits, the effect (of the code) is the same as a step motor failure in the sense that the PCM and/or TCM (Transmission Control Module) cannot control the transmission effectively. When this happens, the PCM and/or TCM will set code P1778 and may or may not also illuminate a warning light.

Where is the P1778 sensor located?

This image shows the location (arrowed) of the step motor where it attaches to the valve body of the CVT transmission in a 2008 Nissan Sentra. Note, though, that in almost all Nissan applications, it is necessary to remove the valve body from the transmission to gain access to the step motor.

This requires major disassembly of the transmission, which we recommend novice DIY mechanics do NOT attempt due to the high likelihood that additional damage to the transmission might occur. Instead, we recommend that you seek professional assistance with the diagnosis and repair of code P1778 on all Nissan applications.

Be aware, though, that many repair shops and even Nissan dealerships will recommend or even insist on replacing the entire transmission when code P1778 is present on a Nissan vehicle, even though high-quality and even OEM-equivalent replacement step motors and ratio control valves are freely available in the aftermarket at a minute fraction of the cost of a rebuilt CVT transmission.

What are the common causes of code P1778?

Common causes of code P1778 could include one or more of the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the CVT transmission’s wiring harness
  • Low transmission fluid level
  • Clogged or dirty internal transmission fluid filter
  • Defective or malfunctioning step motor
  • Defective or malfunctioning ratio control valve
  • Defective or malfunctioning pressure pump in the transmission
  • Excessive mechanical wear in the transmission that causes a loss of line pressure
  • The use of unsuitable or incorrect transmission fluid
  • The use of dirty, contaminated, or severely degraded transmission fluid
  • The use of an unsuitable or incompatible step motor or ratio control valve
  • The use of some aftermarket step motors or ratio control valves
  • The use of an incompatible PCM
  • The use of an incompatible TCM
  • Corrupted programming in either, or both the PCM and TCM

What are the symptoms of code P1778?

The most common symptoms of code P1778 are similar across all Nissan applications and could include one or more of the following-

  • Stored trouble code and possibly an illuminated warning light
  • Depending on the nature of the problem, multiple additional codes may be present along with P1778
  • The vehicle may not respond to throttle inputs
  • Take-offs from a standing start could be harsh, or even uncontrollable
  • The transmission may be stuck in any “gear ratio”
  • The transmission may not select “R”
  • The transmission may not select “D”
  • The transmission may not select “N”
  • The transmission may not “shift” gears as it normally did
  • “Gear shifts” may be unpredictable

In rare cases, the drive belt may slip, causing it to overheat to the point of failure

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