Codes: How To Retrieve??
Thread Starter
Burning Brakes
Joined: Aug 2004
Posts: 771
Likes: 0
From: Washington District of Columbia
Codes: How To Retrieve??
How does one retrieve codes to troubleshoot engine problems? Is there
special equipment needed, etc.? In my case for a '94 LT1. TIA.
special equipment needed, etc.? In my case for a '94 LT1. TIA.
Melting Slicks
Joined: Jan 2008
Posts: 2,065
Likes: 1
From: Jasper Texas
this is all I've got:
For 1994 and later models, you will need to recover the codes using a special procedure involving the speedometer and the odometer as explained in the service manual or use a special diagnostic device known as a scan tool.
For 1994 and later models, you will need to recover the codes using a special procedure involving the speedometer and the odometer as explained in the service manual or use a special diagnostic device known as a scan tool.
Safety Car
Joined: Jun 2006
Posts: 3,629
Likes: 28
From: FL
Pulling Codes Revisited
________________________________________
From 1984 until the end of the 1993 model year a 12 pin ALDL was used. After that, a 16 pin ALDL connector was used. The 1994 and 1995 model year still used the OBD-I system even though they have 16 pin connectors. The 16 pin connector in the 1996 C4 is used for the much more complex OBD-II system and a scan tool is required to discover the OBD-II system's secrets.
For the '84- end of '93, 12 pins:
Pin "B" is the diagnostic enable pin and pin "A" is ground. Grounding pin "B" to enable the diagnostic readout of the ECM/PCM.
To recover the codes, short pins "A" and "B" together using a small section of electrical wire or paper clip.
With the ignition turned OFF, short pins "A" and "B" on the ALDL.
Turn IGN ON (but not to RUN).
The "Check Engine" light (early C4s) or "SYS" light (later C4s) will flash a Code 12 (a single flash followed by two flashes) and will repeat three times (Flash (pause) Flash Flash (long pause), Flash (pause) Flash Flash (long pause), Flash (pause) Flash Flash (long pause).
Code 12 is a delimiter or marker code to show where the error code string begins and ends.
After the three Code 12 flashes, you will either get an error code (or codes) or you will get another string of Code 12 flashes if there are no trouble codes stored.
All codes are repeated three times with a long pause between each code group (36 or Flash Flash Flash pause followed by six flashes, repeated three times, folowed by a long pause with any additional codes stored then flashed).
The "Check Engine/System" light on with engine running means the condition(s) are currently present. If the light is not on during RUN operation, the limits were exceeded at some point in time and the event was recorded in memory, but the reading has since returned to the normal operating range.
Remember to remove the shorting device from the connector after you have read the codes.
Clearing the Codes
To clear the codes from memory, remove the negative battery cable for a minimum of 10 seconds.
Disconnecting the battery will clear all stored codes and and any stored memory (radio button presets, clock, trip odometer, average gas mileage memory, power seats). Your ECM/PCM computer will also have to relearn timing/mixture/exhaust emissions.
Make absolutely certain the ignition key is turned OFF. If you connect the battery with the ignition switch ON, you can destroy the ECM/PCM module.
For 1994 and later models, you will need to recover the codes using a special procedure involving the speedometer and the odometer as explained in the service manual or use a special diagnostic device known as a scan tool.
For the '94 - '95, 16 pins (but still OBD I):
Beginning in 1994, you short pin "12" to pin "4" and once again turn the ignition switch to "On" without starting the engine.
The speedometer will display any codes using the same protocol as the 1990-1993 model but there are some changes in the designation for the modules plus additional modules are added:
Module "1" is still the CCM module.
Module "4" is now called the PCM module (Powertrain Control Module) because automatic transmission computer control was added to Engine Control Module in 1994.
Module 7 (on the 1996 model only) is the RTD module. (This is the Real Time Dampening module which replaced the Selective Ride Control module in 1996).
Module 9 is the ABS/ASR module number from 1994 through 1996.
Finally, Module "A", the DERM (Dynamic Energy Control Module --- the air bag control module) will be requested to show any codes.
Just like the 1990-1993 display, you read the codes on the speedometer and read the module number on the trip odometer.
Again, you can cause the codes to repeat by turning the ignition off for five seconds and then back on.
Turn the ignition off and remove the short to restore normal operation.
1996 to 2004 Corvette (OBD II):
Turn IGN ON, ENG OFF.
ON DIC, Press RESET button to clear any warning messages.
Press and hold OPTIONS.
While holding OPTIONS, press FUEL four times within 10 sec.
Go into an AUTOMATIC mode to shows all parameters: PCM-TCS-RTD-BCM-IPC-RADIO-HVAC-LDCM-RDCM-SCM-RFA. If there are no problems, you will see "NO MORE CODES" on the ICP display.
The computer displays two kinds of codes, CURRENT and HISTORY, designated "C" or "H". A CURRENT code indicates a malfunction is present in the system whose module is displaying data. A HISTORY code indicates a past problem.
When the screen displays MANUAL DIAGNOSTICS, select the desired module by pressing the OPTIONS button to go forward or the TRIP button to go back. Press GAUGES to go forward or FUEL to go back.
To exit the diagnostic mode at any time, press E/M. If you want to erase or CLEAR codes, press RESET.
________________________________________
From 1984 until the end of the 1993 model year a 12 pin ALDL was used. After that, a 16 pin ALDL connector was used. The 1994 and 1995 model year still used the OBD-I system even though they have 16 pin connectors. The 16 pin connector in the 1996 C4 is used for the much more complex OBD-II system and a scan tool is required to discover the OBD-II system's secrets.
For the '84- end of '93, 12 pins:
Pin "B" is the diagnostic enable pin and pin "A" is ground. Grounding pin "B" to enable the diagnostic readout of the ECM/PCM.
To recover the codes, short pins "A" and "B" together using a small section of electrical wire or paper clip.
With the ignition turned OFF, short pins "A" and "B" on the ALDL.
Turn IGN ON (but not to RUN).
The "Check Engine" light (early C4s) or "SYS" light (later C4s) will flash a Code 12 (a single flash followed by two flashes) and will repeat three times (Flash (pause) Flash Flash (long pause), Flash (pause) Flash Flash (long pause), Flash (pause) Flash Flash (long pause).
Code 12 is a delimiter or marker code to show where the error code string begins and ends.
After the three Code 12 flashes, you will either get an error code (or codes) or you will get another string of Code 12 flashes if there are no trouble codes stored.
All codes are repeated three times with a long pause between each code group (36 or Flash Flash Flash pause followed by six flashes, repeated three times, folowed by a long pause with any additional codes stored then flashed).
The "Check Engine/System" light on with engine running means the condition(s) are currently present. If the light is not on during RUN operation, the limits were exceeded at some point in time and the event was recorded in memory, but the reading has since returned to the normal operating range.
Remember to remove the shorting device from the connector after you have read the codes.
Clearing the Codes
To clear the codes from memory, remove the negative battery cable for a minimum of 10 seconds.
Disconnecting the battery will clear all stored codes and and any stored memory (radio button presets, clock, trip odometer, average gas mileage memory, power seats). Your ECM/PCM computer will also have to relearn timing/mixture/exhaust emissions.
Make absolutely certain the ignition key is turned OFF. If you connect the battery with the ignition switch ON, you can destroy the ECM/PCM module.
For 1994 and later models, you will need to recover the codes using a special procedure involving the speedometer and the odometer as explained in the service manual or use a special diagnostic device known as a scan tool.
For the '94 - '95, 16 pins (but still OBD I):
Beginning in 1994, you short pin "12" to pin "4" and once again turn the ignition switch to "On" without starting the engine.
The speedometer will display any codes using the same protocol as the 1990-1993 model but there are some changes in the designation for the modules plus additional modules are added:
Module "1" is still the CCM module.
Module "4" is now called the PCM module (Powertrain Control Module) because automatic transmission computer control was added to Engine Control Module in 1994.
Module 7 (on the 1996 model only) is the RTD module. (This is the Real Time Dampening module which replaced the Selective Ride Control module in 1996).
Module 9 is the ABS/ASR module number from 1994 through 1996.
Finally, Module "A", the DERM (Dynamic Energy Control Module --- the air bag control module) will be requested to show any codes.
Just like the 1990-1993 display, you read the codes on the speedometer and read the module number on the trip odometer.
Again, you can cause the codes to repeat by turning the ignition off for five seconds and then back on.
Turn the ignition off and remove the short to restore normal operation.
1996 to 2004 Corvette (OBD II):
Turn IGN ON, ENG OFF.
ON DIC, Press RESET button to clear any warning messages.
Press and hold OPTIONS.
While holding OPTIONS, press FUEL four times within 10 sec.
Go into an AUTOMATIC mode to shows all parameters: PCM-TCS-RTD-BCM-IPC-RADIO-HVAC-LDCM-RDCM-SCM-RFA. If there are no problems, you will see "NO MORE CODES" on the ICP display.
The computer displays two kinds of codes, CURRENT and HISTORY, designated "C" or "H". A CURRENT code indicates a malfunction is present in the system whose module is displaying data. A HISTORY code indicates a past problem.
When the screen displays MANUAL DIAGNOSTICS, select the desired module by pressing the OPTIONS button to go forward or the TRIP button to go back. Press GAUGES to go forward or FUEL to go back.
To exit the diagnostic mode at any time, press E/M. If you want to erase or CLEAR codes, press RESET.
Melting Slicks
Joined: Oct 2004
Posts: 2,510
Likes: 4
From: Austin Texas
This is the same info that toptech posted, but I find this to be a little easier on the eyes....
http://www.corvettebuyers.com/c4vettes/codes.htm
and here is a good site containing the secret decoder ring for error codes:
http://www.troublecodes.net/GM/
http://www.corvettebuyers.com/c4vettes/codes.htm
and here is a good site containing the secret decoder ring for error codes:
http://www.troublecodes.net/GM/
Last edited by MikeC4; Feb 18, 2009 at 04:52 PM.
Safety Car
Joined: Jun 2006
Posts: 3,629
Likes: 28
From: FL
This is the same info that toptech posted, but I find this to be a little easier on the eyes....
http://www.corvettebuyers.com/c4vettes/codes.htm
and here is a good site containing the secret decoder ring for error codes:
http://www.troublecodes.net/GM/
http://www.corvettebuyers.com/c4vettes/codes.htm
and here is a good site containing the secret decoder ring for error codes:
http://www.troublecodes.net/GM/
The OP also needs to know whatever codes are retrieved do not necessarily mean the item associated with the code (sensor or output device) is bad. Proper diagnosis requires following the diagnostic procedure in the FSM.
