Bulletin No.:

Service Bulletin

21-NA-004

Date:

January, 2021

INFORMATION
Subject:

Information on Transmission Flares, Slipping, Gear Hunting, Harsh Shifting, Only
Shifts at Wide Open Throttle (WOT) or Hard Acceleration, Rolling in Reverse or in
Wrong Direction Selected, High Engine RPM, Shift Busyness, Surging While Cruising,
Will Not Upshift, Holds in a Lower Gear, Will Not Back Up Hills, Not Shifting, Jerks and/
or Loss of Power
Model Year:

Brand:

Model:

Buick

Encore GX

2020

Malibu

2019

Trailblazer
(VIN M)

2021

Chevrolet

from

VIN:
to

from

to

Engine:

Transmission:

1.2L (LIH) or
1.3L (L3T)
2021

—

—

1.5L (LFV)
1.2L (LIH)
or L3T

Involved Region or Country

North America

Condition

Some customers may comment on one or more of the following conditions:
• RPM flare, slipping and hunting
• Harsh shifts, shifting hard, only shifts at WOT or hard acceleration
• Rolling Reverse to Drive or rolling in wrong direction
• High engine RPM, shift busyness, surging with cruise
• Will not upshift
• Holds lower gear
• Will not back up hills/difficult to accelerate in Reverse
• Not shifting or slipping
• Not shifting, jerk, traction control, loss of power

Cause

These conditions may be a normal operation of a Continuously Variable
Transmission (CVT).

Information
RPM Flare, Slipping and Hunting
The CVT allows the engine to operate in a range that is
the best fuel economy and emissions, while still
delivering the power requested by the driver. When the
driver is at very light pedal or holding steady speeds on
flat roads, the transmission will operate as close to full
overdrive as it can which keeps the engine RPMs low.
As power needs to be increased (driver steps-in to the
accelerator, a slight incline is reached, etc.), a small
boosted engine can make this power at a large range of
speeds due to the added capability of the turbo.
However, the most efficient area to operate the engine

Copyright 2021 General Motors LLC. All Rights Reserved.

VT40 CVT
Automatic
(MRG)

in these cases is at higher engine RPMs and less
turboboost. This provides the same power, but at a
much more efficient operating condition, saving fuel
and improving emissions. While the CVT allows these
swings in engine RPM to happen seamlessly, a driver
may comment that this is different than a step gear
transmission that may hold gears rather than shifting.
This can result in comments of “Engine Flare”,
“Transmission Slipping”, or “Transmission Hunting.”
This is normal to the CVT and enables the same overall
vehicle performance as a step-gear transmission, with
the added benefit of fuel savings and lower emissions.

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January, 2021

Harsh Shifts, Shifting Hard, Only Shifts at WOT
or Hard Acceleration
While most drivers operate their vehicle at low to mid
accelerator pedal positions. Some may notice the CVT
operates differently at higher pedals. The CVT uses
“Continuous Ratio Control” while driving < 70%
accelerator pedal. This means the engine RPMs can
fluctuate freely from ~1000-4000 RPM in order optimize
fuel economy and emissions while still delivering the
power needed for the current driving conditions. As the
driver steps into >70% Accelerator Pedal, the
transmission control enters a performance mode. This
will limit the continuous ratio control and allow the
transmission to operate much like a step-gear
transmission to give the feel of performance. When
entering this mode, the engine will rise up to and RPM
higher than 4000 RPM, and will upshift at higher RPMs
with higher accelerator pedal positions. Once the driver
lifts out of the pedal, normal continuous ratio control will
resume.

Rolling Reverse to Drive or Rolling in Wrong
Direction
Some customers may note that the vehicle can roll
backwards while in a forward gear or roll forward while
in Reverse gear. Due to the small engine displacement
and the slip available within the torque converter, the
amount of torque to the drive wheels at idle is low. This
means the vehicle could roll on a grade from a stop or
may continue to roll in the previous direction when
doing direction change shift (D->R, R->D). Brakes
should be applied when the vehicle is on a hill to
prevent unwanted movement.

High Engine RPM, Shift Busyness, Surging
With Cruise
The CVT allows the engine to operate in a range that is
the best fuel economy and emissions, while still
delivering the power requested by the driver. When
holding steady speeds on flat roads, the transmission
will operate as close to full overdrive as it can which
keeps the engine RPMs low. As power needs to be
increased (a slight incline is reached, rolling hills, driver
commands a higher cruise control speed, etc.), a small
boosted engine can make this power at a large range of
speeds due to the added capability of the turbo.
However, the most efficient area to operate the engine
in these cases is at higher engine RPMs and less turbo
boost. This provides the same power, but at a much
more efficient operating condition, saving fuel and
improving emissions. While the CVT allows these
swings in engine RPM to happen seamlessly, a driver
may comment that this is different than a step gear
transmission that may hold gears rather than shifting.
This can result in comments of “High Engine RPM”,
“Shift Busyness”, or “Cruise Control Surging”, but this is
normal to the CVT and enables the same overall
vehicle performance as a step-gear transmission, with
the added benefit of fuel savings and lower emissions.

Bulletin No.: 21-NA-004

Will Not Upshift
The CVT utilizes a Manual Range shifter much like a
step-gear transmission. In this mode, the transmission
will hold the selected range and will limit the top gear to
what is shown on the dash. If the driver accidentally
pulls the shifter in this mode, it will force the
transmission to limit the ratio based on the gear
selected and this could result in the engine RPMs to
reach red-line and prevent the vehicle from reaching
higher speeds. This operation is the same for any GM
transmission featuring Manual Range and is working as
designed.

Holds Lower Gear
When operating the vehicle on steeper grades (incline
or decline), a driver may notice that the transmission
may not upshift to full overdrive as it does when
operating on flat ground. This is the intended operation
to prevent the transmission from “hunting” and helps
keep the engine rpms higher to reduce turbo lag and
respond quickly to changing pedal positions. This is
especially helpful on rolling hills where the grade can
change often.

Will Not Back Up Hills/Difficult to Accelerate in
Reverse
Some customers may comment that the vehicle does
not back up a steep hill or over is difficult to accelerate
in Reverse. This is due to the gear ratio being less
aggressive in Reverse than in forward 1st gear. Even
though Reverse has less mechanical advantage, the
driver can step-in to the pedal further and the engine
will provide enough torque to accelerate up grades and
over obstacles in Reverse. This operating condition is
normal.

Not Shifting or Slipping
The CVT uses “Continuous Ratio Control” while driving
< 70% Accelerator Pedal. This means the engine
RPMs can fluctuate freely from ~1000-4000 RPM in
order optimize fuel economy and emissions while still
delivering the power needed for the current driving
conditions. This can sometimes be noted as the
transmission “slipping” or even “not shifting” due to its
continuous change in engine RPM without a distinct
shift feel. When holding steady speeds on flat roads,
the transmission will operate as close to full overdrive
as it can which keeps the engine RPMs low. As power
needs to be increased (driver steps-in to the
accelerator, a slight incline is reached, etc.), a small
boosted engine can make this power at a large range of
speeds due to the added capability of the turbo.
However, the most efficient area to operate the engine
in these cases is at higher engine RPMs and less turbo
boost. This provides the same power, but at a much
more efficient operating condition, saving fuel and
improving emissions. While the CVT allows these
swings in engine RPM to happen seamlessly, a driver
may comment that this is different than a step gear
transmission that may hold gears rather than shifting.
These changes in engine rpm may be noticed at slight
changes to the accelerator pedal and is expected with
the CVT.

Bulletin No.: 21-NA-004

January, 2021

Page 3

Not Shifting, Jerk, Traction Control, Loss of
Power
During aggressive driving around corners or while
driving on different slippery and/or rough surfaces, the
driver may notice that the vehicle may hesitate or not
shift as expected. During wheel slip conditions, there
are several systems that protect the transmission and
prevent the vehicle from losing control. During these
events, the Traction Control system or the transmission
may command different levels of torque management
until the vehicle is under control and/or the wheels
show no sign of slip. The CVT requires a higher level of
hardware protection than a step-gear transmission and
this type of operation may be more noticeable in the
above listed conditions. There should be no noticeable
torque control intervention during normal driving
conditions.

Parts Information
No parts are required for this repair.

Version

1

Modified

Released January 12, 2021

GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these
technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper
service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a
job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your
vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the
information.

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Bulletin No.: 21-NA-004

January, 2021

Page 3