Bulletin No.:

Service Bulletin

Date:

19-NA-253
November, 2019

TECHNICAL
Subject:

Diagnostic Tip for Engine Misfire, Malfunction Indicator Lamp (MIL) Illuminated DTCs P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307 and/or P0308

Brand:

Model:

Cadillac

CT6

Model Year:

VIN:

from

to

2019

2020

from

to

Engine:

Transmission:

4.2L (LTA)

Involved Region or Country

United States, Canada, Mexico, N.A. Export Regions

Condition

Some customers may comment on one or more of the following conditions:
• MIL illuminated
• Misfire
Some technicians may find one or more of the following DTCs set in the Engine Control
Module (ECM):
• P0300
• P0301
• P0302
• P0303
• P0304
• P0305
• P0306
• P0307
• P0308

Cause

This condition may be caused by one or more of the following:
• Fuel Injector
• Electrical issue
• Loss of cylinder compression

Correction

Note: The purpose of the bulletin is to show you a way to confirm if you have a
mechanical engine issue or not.
A Relative Compression test can be used to determine if the misfire is caused by an
engine compression issue or not.

Service Procedure for Running a
Compression Test
A technician may find that a relative compression test
can be used to rule out an engine compression issue. A
relative compression test can be performed with
different tools and the engine can be cranked over
using many different methods.

Copyright 2019 General Motors LLC. All Rights Reserved.

For this communication, use the PicoScope CH-51450
and available CH-51450-LEAD kits or equivalent.
1. Connect the PicoScope to the computer using the
included USB cable as you would do using
GM NVH.
2. Connect test leads to the battery (hook the colored
lead to Battery (+) and the black lead to Battery (-))
and to channel A of the PicoScope.

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3. Open the most current version of PicoScope 6
Automotive for GM software that is loaded when
GM NVH software is installed.
⇒ If PicoScope 6 Automotive is not on your
computer, download the newest version of Pico
Scope NVH Software Update
V.R.1.11.1_September_2018 or later.

5419699

4. With the PicoScope hooked up and the software
running, a steady line at battery voltage on the
graph should be displayed.

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5. Select the Collection Time drop down.

5419718

6. Change the collection time from 5ms/div to
200ms/div.

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7. While holding the accelerator pedal to the floor
(Clear Flood), crank the engine for two to three
seconds.
8. Immediately after cranking the engine, select the
Stop capturing button at the bottom of the screen.

5419733

9. Save this file to the PC.
⇒ It may be useful to repeat steps 8 thru 10 a few
more times to collect additional wave forms. To
do this, you will need to hit the Go or Start
capturing button prior to cranking the engine.
10. Disconnect the PicoScope.

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Setting Up PicoScope 6 Software to
View Waveforms

5419752

To examine one of these wave forms, use the
Waveform Buffer Index navigation buttons to locate the
start of the cranking.

5419756

The graphic above depicts a viewable Wave Form
Pattern.
To view the Wave Form, zoom in to the right side of the
graph.

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Change settings.
⇒ Select the drop down next to the blue “A” (for
channel A).

5419767

The channel options screen should be displayed.
⇒ Select the Lowpass Filtering Active Filtering check
box (100 Hz to start).
⇒ Change the Axis Scaling Scale value to 26.00.

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Take note, the voltage graph appears to have
disappeared, but if you look at the scaling on the left,
you will see that the value is a negative number.
⇒ Use the axis scaling offset to bring the graph back
into view.

5437335

The graphic above depicts a viewable Voltage Graph
Pattern.

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Use the waveform Buffer button to move to the next
window.

5437339

The graph above illustrates an engine that will pass a
compression test and there is no reason to think that
there is anything mechanically wrong with the engine.
⇒ If the graph does not look like the graph above,
refer to the Analyzing Relative Compression
Graphs section below.

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Analyzing Relative Compression
Graphs

5437350

When examining these graphs, view the downward
portion of the waveform.
This area of the graph is the engine starter pulling more
electricity from the battery during a compression stroke.

5437353

Utilizing a horizontal ruler, line up the bottom of the
waveform.
⇒ Select the blue box and drag it to the location that's
viewable.

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Vertical rulers may be used as well.

5437360

Utilize a second ruler.

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Select the Ruler Settings Button to display the Ruler
Setting menu options.

5437366

Change the rotation partition to 8, to represent the
number of cylinders in the engine.

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This should help to identify the top of each compression
stroke in two rotations of the engine.
It is not an issue if you see a few of these “tips” that do
not touch the line. What we are looking for is
consistently one of every eight points not hitting the line
for an eight cylinder engine.

Sample Waveforms for Reference

5420017

The graphic above depicts an example of an
eight-cylinder engine with a cylinder that has no
compression in one cylinder.

Note that one of the dips is missing and it repeats every
eight dips.
– This is a clear indication that there is an issue with
this engine.

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The graphic above depicts an example of a
four-cylinder engine with a bad exhaust cam lobe.
Note that consistently there is one cylinder with an
extra-long compression stroke.
– Examination of this engine showed that cylinder
number four had a worn down cam lobe.

5420028

This is a four-cylinder engine with a 20 PSI loss in one
cylinder.

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This is a four-cylinder engine with a 40 PSI loss in one
cylinder.

5420040

This is a four-cylinder engine with a 60 PSI loss in one
cylinder.

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If an engine has low or high compression on one
cylinder, it may be useful to add an injector pulse or a
Camshaft Position Sensor to help determine which
cylinder has an issue.
The graphic above shows the signal from the even
bank intake Cam Position Sensor for reference.

Background Information
By graphing the battery voltage while cranking an
engine, we can get a good picture of how the
compression in each cylinder compares to the
compression in each of the other cylinders in the
engine. When you think about an engine being cranked
over by the starter, each time a piston compresses the
charge in the cylinder, the drag on the starter increases,
which in turn, increases the starter draw and decreases
the battery voltage until the air in the cylinder is
compressed.

After this compression stroke and before the next
cylinder in the firing order begins its compression
stroke, there is a short period of cranking where the
voltage increases. This is what gives the us the pattern
to examine on a scope as well as the distinctive sound
of an engine being cranked.
If each cylinder in the engine are of very close
compression, the resulting pattern on the scope will be
equal all the way across the screen. If one of the
cylinders in a four-cylinder engine has low compression
compared to the other three, then every four “waves”
will look different from the other three. The same can be
said of an eight-cylinder engine that has a weak
cylinder. The wave pattern will look different on every
eighth wave.

Version

1

Modified

Released November 12, 2019

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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