For owners of classic 1987-1995 Nissan Pathfinders and Hardbody pickups, diagnosing engine issues might seem daunting in the age of modern OBD-II scanners. However, these robust vehicles have a built-in diagnostic system that utilizes LEDs on the Engine Control Unit (ECU) to communicate Diagnostic Trouble Codes (DTCs). This “Led Link For Dtc Code” provides a straightforward way to understand what’s going on under the hood without needing specialized tools. This guide will walk you through accessing your ECU, interpreting the LED flashes, and understanding the diagnostic modes available in your Nissan.
Locating and Accessing Your Nissan ECU
The first step in utilizing your “led link for dtc code” is to locate the ECU. In 1987-1995 Nissan Pathfinders and Hardbody pickups, the ECU is situated under the passenger side seat.
To access it:
- Move the Passenger Seat: Slide the passenger seat all the way forward to maximize space.
- Remove the Plastic Cover: Look for a plastic cover on the floor under the seat. This cover protects the ECU. It’s typically held in place by a single screw. Remove this screw and then gently pull off the plastic cover.
- Optional: Seat Removal: For easier access, you can remove the passenger seat completely. This isn’t strictly necessary but can provide more room to work.
Once the cover is removed, you’ll have a clear view of your Nissan’s ECU.
Understanding the Diagnostic Mode Selector Switch
To activate the “led link for dtc code” system, you need to engage the diagnostic mode selector switch on the ECU. The type of switch varies slightly depending on your engine type (VG30i, VG30E, or 4-cylinder):
- VG30i ECU: These ECUs feature a small toggle switch on the side, protected by a snap-in cover. You’ll need to remove this cover to access the toggle switch.
- VG30E and 4-cylinder ECUs: These models typically use a slotted rotary switch. This switch is turned clockwise and counterclockwise to select diagnostic modes. Some early 4-cylinder Hardbody ECUs might also have the toggle switch.
Regardless of the type, the function is the same: to put the ECU into diagnostic mode so it can communicate error codes via the LEDs.
Reading DTC Codes Using the LED Link
The “led link for dtc code” manifests as two LEDs on top of the ECU: a red LED and a green LED. These LEDs will flash in patterns to indicate DTCs. The process for reading these codes differs slightly between 4-cylinder and V6 engines.
Procedure for 4-Cylinder Engines:
- Ignition ON (Engine OFF): Turn your ignition key to the “ON” position, but do not start the engine. All dashboard lights should illuminate.
- Engage Diagnostic Mode: Turn the ‘diagnostic mode selector’ screw on the ECU fully clockwise. The check engine light on your dashboard should turn off. Wait for 5 seconds.
- Initiate Code Output: Turn the diagnostic mode selector screw fully counter-clockwise.
- Read the Codes: The check engine light will now begin to flash the trouble code. The number of long flashes represents the first digit of the code, and the number of short flashes represents the second digit. For example, three long flashes followed by two short flashes indicate code 32.
- Clear Codes (Optional): To clear stored codes and turn off the check engine light, turn the screw fully clockwise while it’s in diagnostic mode (flashing codes). Wait 5 seconds, then turn the screw fully counter-clockwise.
- Keep Codes Stored: To retain the stored codes, simply turn the ignition key off while the check engine light is flashing. The ECU will keep the codes, and the check engine light will remain on when you next start the vehicle (if the fault persists).
Procedure for V6 Engines (VG30i and VG30E):
- Ignition ON (Engine OFF): Turn the ignition key to the “ON” position without starting the engine. Ensure all dashboard lights are on.
- Enter Diagnostic Mode:
- VG30i: Locate the small plastic tab on the side of the ECU. Remove it to expose the toggle switch. Use a small screwdriver or tool to move the toggle switch to the “DIAGNOSTICS” position. The red and green LEDs on top of the ECU will begin to blink simultaneously, indicating the diagnostic mode number.
- VG30E (Slotted Switch): Turn the ‘diagnostic mode selector’ screw on the ECU fully clockwise. The check engine light will go out. Wait 5 seconds, then turn the screw fully counter-clockwise.
- Select Mode III (for DTCs): For VG30i, the LEDs will blink to indicate the mode number (1 through 5). Mode 3 is typically used for retrieving DTCs. The LEDs will blink together three times to indicate Mode III. After it blinks three times, move the toggle switch back to the “ON” position. For VG30E, the system should directly proceed to outputting codes after step 2.
- Read the Codes: After selecting Mode III (for VG30i) or initiating code output (for VG30E), the LEDs will start flashing DTCs. The red LED flashes for the tens digit, and the green LED flashes for the units digit. For example, one red flash followed by two green flashes indicates code 12. Record each code as it is displayed.
- Clear Codes (V6): To clear trouble codes on a V6, move the switch to the “DIAGNOSTICS” position and then back to the “ON” position, and finally turn off the ignition key.
1986.5-1995 Nissan ECU Flash Codes (All Engines)
Once you have retrieved the flash codes using your “led link for dtc code”, you need to interpret them. Here is a comprehensive list of ECU flash codes for 1986.5-1995 Nissan vehicles:
| Code | Description | Code | Description |
|---|---|---|---|
| 11 | Crank Angle Sensor/Camshaft Position Sensor | 55 | No Malfunction |
| 12 | Air Flow Meter/Mass Air Flow Sensor | 63 | No. 6 Cylinder Misfire |
| 13 | Engine Coolant Temperature Sensor | 64 | No. 5 Cylinder Misfire |
| 14 | Vehicle Speed Sensor | 65 | No. 4 Cylinder Misfire |
| 21 | Ignition Signal | 66 | No. 3 Cylinder Misfire |
| 22 | Fuel Pump | 67 | No. 2 Cylinder Misfire |
| 23 | Idle Switch | 68 | No. 1 Cylinder Misfire |
| 24 | Throttle Valve Switch | 71 | Random Misfire |
| 25 | Idle Speed Control Valve | 72 | TWC Function right hand bank |
| 28 | Cooling Fan Circuit | 73 | TWC Function right hand bank |
| 31 | ECM (Engine Control Module) | 76 | Fuel Injection System Function right hand bank |
| 32 | EGR Function | 77 | Rear Heated Oxygen Sensor Circuit |
| 33 | Heated Oxygen Sensor | 82 | Crankshaft Position Sensor |
| 34 | Knock Sensor | 84 | A/T Diagnosis Communication Line |
| 35 | Exhaust Gas Temperature Sensor | 85 | VTC Solenoid Valve Circuit |
| 36 | EGR Control-Back Pressure Transducer | 86 | Fuel Injection System Function right hand bank |
| 37 | Knock Sensor | 87 | Canister Control Solenoid Valve Circuit |
| 38 | Right hand bank Closed Loop (B2) | 91 | Front Heated Oxygen Sensor Heater Circuit right hand bank |
| 41 | Intake Air Temperature Sensor | 94 | TCC Solenoid Valve |
| 42 | Fuel Temperature Sensor | 95 | Crankshaft Position Sensor |
| 43 | Throttle Position Sensor | 98 | Engine Coolant Temperature Sensor |
| 44 | ECCS Normal Operation (No Fault) | 101 | Front Heated Oxygen Sensor Heater Circuit right hand bank |
| 45 | Injector Leak | 103 | Park/Neutral Position Switch Circuit |
| 47 | Crankshaft Position Sensor | 105 | EGR and EGR Canister Control Solenoid Valve Circuit |
| 51 | Injector Circuit | 108 | Canister Purge Control Valve Circuit |
| 53 | Oxygen Sensor | ||
| 54 | A/T Control |
ECU Diagnostic Mode Procedures (Modes I – V)
Beyond retrieving stored fault codes, your Nissan ECU offers several diagnostic modes accessible through the selector switch. These modes provide real-time data and component checks.
Mode I – Exhaust Oxygen Sensor Monitor:
- Purpose: To check if the Oxygen Sensor is functioning correctly.
- Procedure:
- Warm the engine to normal operating temperature.
- Run the engine above 2000 RPM (no load).
- Observe the GREEN LED. It should blink ON and OFF more than five times within ten seconds at 2000 RPM.
- Diagnosis: If fewer than five flashes occur, replace the oxygen sensor. If the LED doesn’t flash at all, check the sensor’s circuit.
Mode II – Mixture Ratio Control Monitor:
- Purpose: To determine if the air/fuel mixture is cycling correctly (rich/lean).
- Procedure:
- Warm engine to normal operating temperature.
- For slotted switch: Turn dial fully clockwise, then after 2 flashes, turn fully counter-clockwise. For toggle switch: Turn switch to DIAGNOSTICS, wait for 2 flashes, then return to ON.
- Run engine above 2000 RPM (no load).
- Observe the RED LED.
- Diagnosis:
- RED LED blinks simultaneously with GREEN LED: Air/fuel mixture cycling correctly.
- RED LED stays OFF: Mixture is more than 5% rich.
- RED LED stays ON: Mixture is more than 5% lean.
- If RED LED stays ON or OFF above 2000 RPM, complete full DTC diagnostics before further repairs.
Mode III – Stored Fault Code Reporting:
- Purpose: Retrieves all stored trouble codes (DTCs) in memory.
- Procedure: Use the procedures outlined earlier for 4-cylinder and V6 engines to read DTC codes.
Mode IV – Switch Operation Monitor:
- Purpose: Checks the operation of the Throttle Position Sensor (TPS), Starter circuit, and Vehicle Speed Sensor (VSS).
- Procedure:
- Ignition ON (Engine OFF).
- For slotted switch: Turn dial fully clockwise, then after 4 flashes, turn fully counter-clockwise. For toggle switch: Turn switch to DIAGNOSTICS, wait for 4 flashes, then return to ON.
- TPS Check: Ensure the RED LED is initially OFF. Depress the gas pedal fully. The RED LED should turn ON. If not, check or replace the TPS.
- Starter Circuit Check: Turn the ignition key to the “START” position (briefly). The RED LED should turn ON. If not, check or replace the starter circuit.
- VSS Check: Drive the vehicle with the ECU in view (carefully!). At speeds of 12 MPH or more, the GREEN LED should turn ON. If not, check or replace the VSS.
Mode V – Real-Time Diagnostic Function:
- Purpose: Checks the Crankshaft Position Sensor (CPS), Mass Air Flow Sensor (MAFS), and Ignition signal in real-time while the engine is running.
- Procedure:
- Start the engine.
- For slotted switch: Turn dial fully clockwise, then after 5 flashes, turn fully counter-clockwise. For toggle switch: Turn switch to DIAGNOSTICS, wait for 5 flashes, then return to ON.
- Monitor LEDs: Observe the LEDs for 5 minutes while idling and revving the engine. Ideally, no LEDs should flash during this period.
- Fault Diagnosis (if LEDs flash):
- RED LED flashes 3 times (1.5 sec each): Check Crankshaft Position Sensor (CPS) and its wiring harness for continuity, dust, and pin terminal issues.
- GREEN LED flashes 2-3 times (0.5 sec each): Check Mass Air Flow Sensor (MAFS) and its wiring harness for continuity, dust, and pin terminal issues.
- GREEN LED flashes 4 times (0.2 sec each): Check the ignition signal and its wiring harness for continuity, dust, and pin terminal issues.
By understanding and utilizing this “led link for dtc code” system, owners of 1987-1995 Nissan Pathfinders and Hardbody pickups can effectively diagnose and address engine issues, keeping these classic vehicles running smoothly for years to come. Remember to consult a qualified mechanic if you are unsure about any diagnostic or repair procedures.
