For owners of the rugged and reliable 1991 Nissan D21 Hardbody, troubleshooting engine issues might seem daunting, especially when thinking about expensive scan tools. The good news is, your Hardbody pickup truck is equipped with a built-in diagnostic system that allows you to retrieve ECU error codes without needing a dedicated scan tool. This guide will walk you through accessing your ECU and interpreting the codes, empowering you to diagnose problems and potentially save on repair costs.
Accessing Your Nissan Hardbody ECU for Diagnostics
The Electronic Control Unit (ECU) in your Nissan D21 Hardbody is conveniently located under the passenger side seat. To access it, simply slide the passenger seat all the way forward. You’ll see a plastic cover – this is protecting your ECU. For easier access, you can choose to remove the seat completely, but just sliding it forward usually provides enough room to work.
Once you’ve accessed the ECU, you’ll notice a diagnostic mode selector switch on the side of the unit. The type of switch depends on your engine type:
- VG30i ECU (V6 engine): These ECUs have a small toggle switch with a snap-in cover. This toggle switch selects the diagnostic mode.
- 4-Cylinder ECU & VG30E ECU (V6 engine): These typically have a slotted screw that you turn with a screwdriver to select diagnostic modes. Some early 4-cylinder Hardbody ECUs might also have the toggle switch.
Regardless of the type, your ECU will have one of these switches to initiate the diagnostic process.
Nissan D21 Hardbody ECU located under the passenger seat, accessed by sliding the seat forward and removing the plastic cover.
Retrieving Error Codes: Step-by-Step Guides
Now that you can access your ECU, let’s get into how to retrieve those valuable error codes. The procedure varies slightly depending on whether you have a 4-cylinder or V6 engine.
For 4-Cylinder Nissan Hardbody Pickups:
- Ignition ON, Engine OFF: Turn your ignition key to the “ON” position, but do not start the engine. All your dashboard lights should illuminate.
- Clockwise Turn: Locate the ‘diagnostic mode selector’ screw on your ECU. Using a screwdriver, turn it fully clockwise. The check engine light on your dashboard should turn off.
- Wait and Counter-Clockwise Turn: Wait for 5 seconds. Then, turn the screw fully counter-clockwise.
- Count the Flashes: Watch your check engine light. It will now begin to flash error codes. The codes are communicated through a series of flashes: long flashes represent the first digit of the code, and short flashes represent the second digit. Count them carefully.
- Clearing Codes (Optional): If you want to clear the stored error codes and turn off the check engine light, turn the screw fully clockwise while it is in diagnostic mode (flashing codes). Wait 5 seconds, then turn it back counter-clockwise.
- Saving Codes: To keep the error codes stored in the ECU’s memory (and keep the check engine light on), simply turn the ignition key off while the check engine light is flashing.
For V6 Nissan Hardbody Pickups (VG30i & VG30E):
- Ignition ON, Engine OFF: Turn the ignition key to the “ON” position, engine off. Ensure all dash lights are on.
- Engage Diagnostic Mode:
- VG30i (Toggle Switch): Locate the small plastic tab on the side of the ECU (facing the door). Remove this tab. Behind it, you’ll find a small toggle switch. Use a small screwdriver or tool to move the switch to the “DIAGNOSTICS” position. You’ll see both red and green LEDs on top of the ECU blink simultaneously. They will blink once, pause, then twice, pause, three times, and so on, cycling through modes 1 to 5. Mode 3 is the standard mode for retrieving trouble codes.
- VG30E (Slotted Screw): Turn the ‘diagnostic mode selector’ screw on the ECU fully clockwise. The check engine light should go out. Wait 5 seconds, then turn it fully counter-clockwise.
- Select Mode 3 (VG30i): After the LEDs blink three times simultaneously (indicating Mode 3), move the toggle switch back to the “ON” position.
- Code Output: After about five seconds, the ECU will start outputting diagnostic codes. For V6 engines, codes are displayed using two LEDs on the ECU:
- Red LED: Flashes indicate the “tens” digit of the code.
- Green LED: Flashes indicate the “ones” digit of the code.
- For example, one red flash followed by two green flashes means code 12.
- Clearing Codes (V6): To clear trouble codes on a V6, move the switch to the “DIAGNOSTICS” position, then back to the “ON” position, and finally turn off the ignition key.
Diagnostic selector switch with cover installed on a Nissan Hardbody ECU, showing the location of the switch.
Diagnostic selector switch with the cover removed and placed on top of the ECU, revealing the toggle switch underneath.
1986.5-1995 Nissan ECU Flash Codes (All Engines)
Once you have retrieved the flash codes, you’ll need to decipher what they mean. Here’s a comprehensive list of ECU error codes for 1986.5-1995 Nissan vehicles, applicable to your 1991 Nissan D21 Hardbody:
| Code | Description | Code | Description |
|---|---|---|---|
| 11 | Crank Angle Sensor/Camshaft Position Sensor | 53 | Oxygen Sensor |
| 12 | Air Flow Meter/Mass Air Flow Sensor | 54 | A/T Control |
| 13 | Engine Coolant Temperature Sensor | 55 | No Malfunction (System OK) |
| 14 | Vehicle Speed Sensor | 63 | No. 6 Cylinder Misfire |
| 21 | Ignition Signal | 64 | No. 5 Cylinder Misfire |
| 22 | Fuel Pump | 65 | No. 4 Cylinder Misfire |
| 23 | Idle Switch | 66 | No. 3 Cylinder Misfire |
| 24 | Throttle Valve Switch | 67 | No. 2 Cylinder Misfire |
| 25 | Idle Speed Control Valve | 68 | No. 1 Cylinder Misfire |
| 28 | Cooling Fan Circuit | 71 | Random Misfire |
| 31 | ECM (Engine Control Module) | 72 | TWC Function right hand bank |
| 32 | EGR Function | 73 | TWC Function right hand bank |
| 33 | Heated Oxygen Sensor | 76 | Fuel Injection System Function right hand bank |
| 34 | Knock Sensor | 77 | Rear Heated Oxygen Sensor Circuit |
| 35 | Exhaust Gas Temperature Sensor | 82 | Crankshaft Position Sensor |
| 36 | EGR Control-Back Pressure Transducer | 84 | A/T Diagnosis Communication Line |
| 37 | Knock Sensor | 85 | VTC Solenoid Valve Circuit |
| 38 | Right hand bank Closed Loop (B2) | 86 | Fuel Injection System Function right hand bank |
| 41 | Intake Air Temperature Sensor | 87 | Canister Control Solenoid Valve Circuit |
| 42 | Fuel Temperature Sensor | 91 | Front Heated Oxygen Sensor Heater Circuit right hand bank |
| 43 | Throttle Position Sensor | 94 | TCC Solenoid Valve |
| 44 | ECCS Normal Operation (No Faults) | 95 | Crankshaft Position Sensor |
| 45 | Injector Leak | 98 | Engine Coolant Temperature Sensor |
| 47 | Crankshaft Position Sensor | 101 | Front Heated Oxygen Sensor Heater Circuit right hand bank |
| 51 | Injector Circuit | 103 | Park/Neutral Position Switch Circuit |
| 52 | Vehicle Speed Sensor | 105 | EGR and EGR Canister Control Solenoid Valve Circuit |
| 108 | Canister Purge Control Valve Circuit |
With this code list, you can now match the flash codes you retrieved to a potential issue in your 1991 Nissan Hardbody.
Exploring ECU Diagnostic Modes for Deeper Analysis
Beyond retrieving stored error codes, your Nissan Hardbody ECU offers several diagnostic modes for more in-depth analysis. These modes help you monitor specific sensor functions and system operations in real-time. To access these modes (Mode I to Mode V), you will use the diagnostic selector switch on your ECU, following a specific sequence of turning the switch or screw.
Mode I – Exhaust Oxygen Sensor Monitor:
This mode verifies if your oxygen sensor is functioning correctly.
- Warm up the engine to its normal operating temperature.
- Run the engine above 2000 RPM (no load).
- Observe the GREEN LED on the ECU. It should flash ON and OFF more than five times within ten seconds.
- If it flashes less than five times, the oxygen sensor might be faulty and need replacement. If it doesn’t flash at all, check the sensor’s circuit for wiring issues.
Mode II – Mixture Ratio Control Monitor:
Mode II checks if the air/fuel mixture is cycling correctly, crucial for efficient engine operation.
- Ensure the engine is at normal operating temperature.
- Engage diagnostic mode by turning the dial fully clockwise (or toggle switch ON).
- After the LEDs flash twice, disengage diagnostic mode by turning the dial fully counter-clockwise (or toggle switch OFF).
- Run the engine above 2000 RPM (no load).
- Observe the RED LED.
- If the RED LED blinks simultaneously with the GREEN LED, the air/fuel mixture is cycling correctly.
- If the RED LED stays OFF, the mixture is too rich (more than 5% rich).
- If the RED LED stays ON, the mixture is too lean (more than 5% lean).
- If the RED LED stays constantly ON or OFF, proceed with further diagnostics before attempting repairs.
Mode III – Stored Fault Code Reporting:
This mode, which we covered earlier, is used to retrieve all stored trouble codes from the ECU’s memory. Follow the “Retrieving Error Codes” steps outlined previously to use Mode III.
Mode IV – Switch Operation Monitor:
Mode IV allows you to test the functionality of the Throttle Position Sensor (TPS), Starter circuit, and Vehicle Speed Sensor (VSS).
- Turn the ignition switch to the “ON” position (engine OFF).
- Engage diagnostic mode by turning the dial fully clockwise. After 4 flashes, disengage by turning counter-clockwise.
- Throttle Position Sensor (TPS) Test: Ensure the RED LED is initially OFF. Press the gas pedal – the RED LED should turn ON when you depress the pedal. If not, inspect or replace the TPS.
- Starter Circuit Test: Turn the ignition key to the “START” position. The RED LED should turn ON. If not, check or replace the starter.
- Vehicle Speed Sensor (VSS) Test: With the ECU accessible and visible, drive the vehicle above 12 MPH. The GREEN LED should turn ON when you reach or exceed 12 MPH. If not, inspect or replace the VSS.
Mode V – Real Time Diagnostic Function:
Mode V is used for real-time diagnosis of the Crankshaft Position Sensor (CPS), Mass Air Flow Sensor (MAFS), and Ignition signal while the engine is running.
- Start the engine.
- Engage diagnostic mode by turning the dial fully clockwise. After 5 flashes, disengage by turning counter-clockwise.
- Observe the LEDs for 5 minutes while idling and revving the engine. Ideally, no LEDs should flash during this time.
- If LEDs flash: Count the flashes, turn off the engine, and refer to the following interpretations:
- RED LED flashes 3 times (1.5 seconds each): Indicates a potential issue with the Crankshaft Position Sensor (CPS). Check the CPS harness for continuity, dust, and pin terminal bends.
- GREEN LED flashes 2-3 times (0.5 seconds each): Points to a possible problem with the Mass Air Flow Sensor (MAFS). Inspect the MAFS harness for continuity, dust, and pin terminal issues.
- GREEN LED flashes 4 times (0.2 seconds each, 3 times consecutively): Suggests an issue with the ignition signal. Check the ignition harness for continuity and terminal problems.
- If no LEDs flash within 5 minutes, turn off the engine – the system is likely functioning correctly for these components.
Location of the Red and Green LEDs on top of the Nissan Hardbody ECU, used to display diagnostic codes and mode indicators.
Conclusion: DIY Diagnostics for Your Hardbody
While modern vehicles rely heavily on sophisticated OBD-II scan tools, your 1991 Nissan D21 Hardbody offers a surprisingly capable built-in diagnostic system. By understanding how to access your ECU, retrieve error codes, and utilize the diagnostic modes, you can gain valuable insights into your truck’s engine health without needing to purchase a separate scan tool. This empowers you to perform initial troubleshooting, potentially saving time and money on repairs. Remember to consult your Nissan Hardbody service manual for detailed repair procedures once you have identified the error codes. Happy diagnosing!
