Accessing & Understanding ECM Trouble Codes
To retrieve trouble codes from your GM fuel injection
system, install a paper clip, jumper wire, or any
conductive tool you can find between the "A" and "B"
terminals on the ALDL diagnostic connector that you
installed in the passenger compartment. You can buy a
metal tool at most auto parts stores for about $2. They
are called code pulling tools. Once you have the jumper
in place, turn the key to the run position, but don't
start the engine. The check engine light will now begin
flashing any trouble codes stored in the ECM's memory.
The codes are flashed 3 times each beginning with the
lowest and continuing to the highest and then the
sequence will start over. The codes are flashed in a
sort of morse code fashion and you have to pay close
attention since if you miss any flashes, you can lose
track and may have to start over. The codes will be
flashed as a sequence of light flashes with short and
long pauses in between the flashes. The long pauses
lasting a second or two signal a new numeral or the
start of another code. The short pauses are just
intervals between flashes to allow you to count the
flashes. For example, a code 12 will look like, flash,
pause, flash, flash. So, what you basically do is count
the flashes to arrive at the numerals that make up a two
digit code. In the previous example, one flash means
numeral 1, followed by a pause, and then two more
flashes means numeral 2. Putting these two numerals
together gives you a code 12. To practice reading codes,
even if the ECM is not telling you of a problem, you can
always get a code 12 with the key in the run position
and the engine off. As a matter of fact, a code 12 will
always be flashed 3 times before any other codes can be
retrieved. The reason is that code 12 means the ECM is
not getting any RPM signal from the distributor (ie.
ignition module). If the engine is not turning, this
will always be the case, so you could say a code 12
means the diagnostics are working as they should. So you
will always get a code 12 even when there is no problem
with the system. I recommend that you practice reading
code 12s early so you aren't forced to learn how to read
codes when you get on the road away from home.
Once you know what codes are stored in the ECM thereby
indicating a problem in the system, you will need to
know what those codes mean. The following list gives a
description of the malfunction associated with each
code. I will add extra info as I see necessary.
Not all of
these codes apply to a TBI system in a Triumph.
||No RPM signals
seen by ECM; if no other codes, all is well with
failure - output remained at .35-.55 volts for
more than a few minutes after warm-up. Possible
broken or burned O2 sensor wire.
failure - indicated temperature above 266F for 3
seconds after engine ran for 20 seconds.
Probably a shorted coolant sensor wire.
failure - indicated temperature below -22F for 3
seconds when engine running over 1 minute.
Probably broken or burned wire.
sensor failure - above 2.5 volts for 2 seconds
when engine speed below 1600 rpm; probably bad
sensor failure - below .2 volt for 2 seconds
while engine running; probably broken or burned
failure - shows < -22 degrees F for 3 seconds
after engine running 1 minute or coolant > 86F.
Not used in TBI systems.
||No speed sensor
pulses when engine between 2000-4000 rpm,
throttle closed, high vacuum, not in neutral and
all for 5 seconds; this code comes on
occasionally as a nuisance, even with a good
sensor; not sure why.
||MAT sensor showed
above 300 degrees F for 2 seconds after engine
ran for over 1 minute. Possible short circuit.
Not used in TBI systems.
||EGR Error Code.
voltage too high (> 4.0 V). Possible vacuum leak
in hose to sensor or faulty sensor.
voltage too low (< 0.25 V) with ignition on or
engine running >1200 rpm and throttle open >20%;
probably broken or burned wire or shorted wire.
idle speed is more than 75 rpm above or below
correct value for more than 45 seconds; probably
bad IAC motor or broken or burned wires to IAC
reference pulses. Ignition voltage < 11 volts
etc.; not used in TBI systems.
||Open or short on
EST or BYPASS line to ignition module; ECM
timing signals not getting to ignition module;
disconnected timing connector will cause this
code or bad ignition module.
Sensor failure - broken, burned or shorted knock
sensor wire or bad sensor.
||O2 sensor showed
< 0.250 volt for over 20 seconds while operating
closed loop - mixture is running lean; possible
clogged fuel filter, fuel tank empty or
sloshing, injectors dirty, fuel pump going bad,
or bad sensor.
||O2 sensor showed
> 0.550 volt for over 50 seconds while in closed
loop with engine running over 1 minute and
throttle open more than 2% - sluggish or dirty
injector, bad O2 sensor.
of EPROM chip in socket and/or bent pins.
||Check that CALPAK
is in place, fully inserted, and no bent pins
has produced >17.1 volts for over 2 seconds.
Check charging system.
||ECU A to D error.
Check ECU grounds, or excessive input voltage.
To clear the
trouble codes, you remove the 20A fuse from the ECM
Special Features of GM
This section describes some
special features of GM fuel injection system and how to
use them. I recommend that you print this section as
well and keep a copy in your vehicle for reference on
Clear Flood Mode - One feature of GM fuel injection is
Clear Flood Mode. This mode stops fuel delivery while
cranking so that any excess fuel build up causing a
flood can be flushed quickly. To activate this mode,
hold the throttle fully to the floor and keep it there
while cranking. The ECM will shut off all fuel flow
until you let off the throttle. But make sure you let
off the throttle if the engine does start. If it was
flooded, it will start once the excess fuel clears.
Field Service Mode - This feature allows the vehicle
operator to check the oxygen sensor feedback system's
condition and verify if the mixture is being controlled
to its optimum value for efficient engine operation. To
enter this mode, place a jumper between ALDL terminals A
and B as in the troubleshooting section above. However,
instead of just turning the key to the run position, go
ahead and start the engine. At first, the check engine
light will flash rapidly about twice a second. This
indicates that the system is in open loop operation and
that the oxygen sensor has not warmed to operating
temperature. A few minutes later, the flashes will
noticeably slow down to about one every two seconds or
even slower. This indicates the system has entered
closed loop operation. At this time, pay close attention
to the amount of time the light remains off and the time
the light stays on. When the light is on, the mixture is
rich. When the light is off, the mixture is lean. As
long as the times of on and off are close to being the
same, the average mixture is at its optimum value. The
reason for this is that standard O2 sensors are very
sensitive at the stoichiometric (ideal for perfect
combustion) air/fuel ratio. So if you approach the ideal
mixture from the rich side, you shoot past the ideal
value into the lean side once you reach that value. If
you approach ideal from the lean side, you again shoot
into the opposite side. As long as you continually
switch back and forth, the average mixture will be
ideal. This operation is due to the limitations of the
inexpensive type of O2 sensor used in today's
automobiles, however, the system works well. There are
more stable types, but they cost hundreds of dollars.
Using this mode on a regular basis will let you know
that your GM fuel injection system is working as it