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GM DTC's (Diagnostic Trouble Codes)



What is a DTC?  A DTC is a diagnostic Trouble Code.  The computers used on Third Generation F-bodies are considered OBD-I (First Generation On Board Diagnostics).  They are called that, because not only do they control the engine management system, but also is capable of performing a self-diagnostics of the engine management.  For example, the ECM (Electronic Control Module) uses the Oxygen Sensor (O2 Sensor) to monitor exhaust gas for being to rich or lean.  If the computer feels that the conditions are right for the sensor to be malfunctioning, it will set a DTC 13 (O2 Sensor circuit fault).  Once a DTC is set, the Check engine light will illuminate.  There are many acronyms for the Check engine light such as CEL, MIL, SES, but we will use CEL (check engine light).  Starting in 1996, OBD-II was introduced.  This is the "second generation On-board diagnostics" 

How do I pull DTC's?  This is simple, on OBD-I vehicles.  You don't even need an expensive scan-tool or code reader, just a simple paper clip.  Locate the ALDL (Assembly Line Diagnostic Lead).  It is usually under the drivers side of the dash.  It may have a cover over it, and may even say "Diagnostic Connector".

This is the ALDL.  The two terminals that you need to be concerned with is in the picture.  It is terminals "A" and "B".  Simply turn the Ignition to "Run" (Do not start the vehicle) and put the paper clip in the two terminals.  Now watch your CEL.  It will start flashing.  It will look like Morse Code, but it isn't, and is real easy to interpret.

The first code that will flash will be a code 12.  It will be  "flash, pause, flash, flash, pause, pause," and repeat the cycle three times.  The ECM will flash each code three times, and move on to the next.  If you have no codes stored, it will keep flashing code 12.  Code 12 simply tells you the ECM is not receiving any pulses from the distributor (since the car is not running).  If you pull codes with a scantool and the car is running, this code will not show up.  Its no big deal, it lets you know the ECM is in the "Field Service Mode"  You will also notice the cooling fans come on, this is normal.

OBD-II codes require a code reader, due to the complexity of the codes.  They are no longer two digit codes, they are 5-character codes, the first character a letter (for example: P0300).  These codes cannot be "flashed" through the check engine light.  The letter stands for the system effected:

"P" codes are Powertrain related, mainly the engine and transmission (i.e. P0201 is a fault with the #1 injector, P1870 is a transmission slip fault)

"B" codes are Body related (Heat/Air Condition, Lamp circuits, etc...)

"C" codes are Chassis related, mainly ABS

"U" codes are communication codes


What do I do with the Codes?  This is less simple.  OBD-I codes are very vague, and diagnosis sometimes isn't that simple.  Most people think that a code 32 (EGR system) means the EGR valve is bad and needs replaced.  This isn't always so!!!  There are many components in the EGR system that could cause the code to set OTHER than the EGR valve itself!  Below is a list of codes and what they mean.


Code:           Meaning:


No Distributor pulses


Oxygen sensor


Coolant Temp sensor circuit


Coolant Temp sensor circuit


Throttle Position Sensor


Throttle Position Sensor


Manifold Air temperature sensor


VSS (Vehicle Speed sensor)


Manifold Air temperature sensor


EGR system


MAP sensor or MAF sensor depending on engine


MAP sensor or MAF sensor depending on engine


Idle Air Control Valve


MAF Burn-off circuit


No Distributor pulses, cylinder select error


Electronic Spark Timing or Bypass fault


Electronic Spark Control fault 


Lean exhaust


Rich exhaust


VATS system fault


PROM fault


CALPAK fault


System over-voltage


Fuel pump circuit


ECM fault


Oxygen sensor fault (degraded sensor)


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