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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.

Code Explanation
12 No RPM signals seen by ECM; if no other codes, all is well with system.
13 Oxygen sensor failure - output remained at .35-.55 volts for more than a few minutes after warm-up. Possible broken or burned O2 sensor wire.
14 Coolant sensor failure - indicated temperature above 266F for 3 seconds after engine ran for 20 seconds. Probably a shorted coolant sensor wire.
15 Coolant sensor failure - indicated temperature below -22F for 3 seconds when engine running over 1 minute. Probably broken or burned wire.
21 Throttle position sensor failure - above 2.5 volts for 2 seconds when engine speed below 1600 rpm; probably bad sensor.
22 Throttle position sensor failure - below .2 volt for 2 seconds while engine running; probably broken or burned wire.
23 MAT sensor failure - shows < -22 degrees F for 3 seconds after engine running 1 minute or coolant > 86F. Not used in TBI systems.
24 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.
25 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.
31 Unknown Error Code.
32 EGR Error Code.
33 MAP sensor voltage too high (> 4.0 V). Possible vacuum leak in hose to sensor or faulty sensor.
34 MAP 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.
35 Closed throttle 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 motor.
41 No Crankshaft reference pulses. Ignition voltage < 11 volts etc.; not used in TBI systems.
42 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.
43 Engine Knock Sensor failure - broken, burned or shorted knock sensor wire or bad sensor.
44 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.
45 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.
51 Check insertion of EPROM chip in socket and/or bent pins.
52 Check that CALPAK is in place, fully inserted, and no bent pins
53 Car's alternator has produced >17.1 volts for over 2 seconds. Check charging system.
54 Unknown error code.
55 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 harness.

Special Features of GM Fuel Injection

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 the road.

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 should.

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