Code 51 - Faulty Prom

Code 51 sets if any of the following occur:

• Faulty PROM unit.
• PROM unit improperly installed (may not set a code if installed backward).
• Some PROM pins not making contact (i.e. bent).

Always check to see that the PROM pins are not bent and inserted properly into ECM. Make sure the PROM is installed in the proper direction as shown in the chart.

Fig 1: Numbered Connector (1 Through 22)

• 1. a. Disconnect the connector from the ECM.
• b. Remove the ECM mounting hardware.
• c. Remove the ECM from the passenger compartment.
• d. Turn the ECM so the bottom cover is facing up.
• e. Remove the slide off PROM access cover with the aid of a small screwdriver placed in the screwdriver slot.
  NOTE: Replacement Electronic Control Module (ECM) is supplied without an engine calibration unit (PROM) so care should be taken when removing the PROM from a defective ECM as it will be reused in the new ECM.
• 2. Grasp the engine calibration unit (PROM) carrier and gently rock the carrier from side to side while applying a firm upward force and remove the PROM.
  Fig 2: Engine Calibration Unit (PROM) Carrier

  
• 3. Take the new electronic control module (if required) out of its packaging and check the service number to make sure it is the same as the defective ECM.
• 4. Take the new engine calibration unit (PROM), (if required) mounted in PROM) carrier, out of its packaging and check the part number making sure it is the same as the one removed from the failed ECM.
• 5. Check PROM mounted in the PROM carrier and make sure the molded rectangular depression on the PROM is at the same end as the 1 and on the PROM carrier. Also make sure there is equal space between the ends of the PROM and the carrier. The carrier has a 1 with a small arrow pointing in the direction of the reference end and where pin one (1) of the PROM is located.
  Fig 3: Carrier Reference End

  
• 6. Position the PROM in the carrier by pushing PROM down until pins are even with the narrow molded offset on the inside wall of the carrier near the bottom end.
  Fig 4: Inside Wall of the Carrier

  
• 7. Position the carrier squarely over the PROM socket with the short reference bar aligned with the short notch in the socket.
  Fig 5: Carrier & PROM Socket

  
• 8. Press down firmly on the top of the carrier.
• 9. While firmly pressing down on the carrier, take a narrow blunt tool and press down on the body of the PROM. Try to seat the PROM in the socket squarely by alternately pressing on either end of it. Since the PROM has 16 pins and the socket has 20 there should be one set of socket pins visible at each end of the PROM.
  Fig 6: Narrow Blunt Tool & Press Down on the Body of the PROM.

  
• 10. a. Replace cover on ECM.
• b. Reinstall cover fastening screw.
• c. Reinstall ECM in passenger compartment.
• d. Connect the connector to the ECM.
• e. Turn ignition on and start engine.
• f. ground diagnostic "test" terminal.
• If trouble code 51 does not occur, the PROM is installed properly.
• If trouble code 51 occurs, the PROM is not fully seated, installed backwards, has bent pin, or is defective.
• If it is necessary to remove the PROM, follow instructions in steps "1" and "2".
• If not fully seated, press firmly on PROM.
• If pins bend, remove PROM, straighten pins and reinstall. If bent pins break or crack during straightening, discard PROM and replace it.
• If found defective, replace PROM.
• If installed backwards, REPLACE PROM.