Decoding Your 1987 Nissan 200SX OBD1 System: A DIY Diagnostic Guide

Navigating the intricacies of your classic 1987 Nissan 200SX can be a rewarding experience for any car enthusiast. However, when the check engine light illuminates, understanding what’s happening under the hood becomes crucial. While modern vehicles boast easily accessible OBD2 ports and scan tools, diagnosing the early OBD1 systems in vehicles like the 1987 Nissan 200SX requires a slightly different approach. This guide will walk you through the process of accessing your Nissan’s Engine Control Unit (ECU) and retrieving error codes without the need for a dedicated 1987 Nissan 200sx Obd1 Scan Tool.

This method, perfectly applicable to a range of Nissan vehicles from this era, including the Pathfinder and Hardbody pickups, empowers you to understand your vehicle’s health directly. By using the built-in diagnostic capabilities of your ECU, you can pinpoint potential issues and take informed steps towards maintenance and repair.

Locating Your Nissan ECU

The first step in accessing your OBD1 diagnostic system is to find your vehicle’s ECU. In Nissan Pathfinders and Hardbody pickups from 1987-1995, and similarly in models like the 1987 Nissan 200SX, the ECU is typically located under the passenger seat.

To gain access:

Slide the passenger seat fully forward. This will provide you with the most working space.
Remove the plastic cover. You’ll find a plastic cover concealing the ECU. This cover is usually secured by a single screw. Remove the screw and gently pull off the cover.

For easier access, you can remove the passenger seat entirely. This provides a completely unobstructed view and makes the process more comfortable, especially if you anticipate spending some time working in this area.

Accessing the ECU under the passenger seat of a Nissan Pathfinder, a similar process for the 1987 Nissan 200SX.

Identifying the Diagnostic Mode Selector Switch

Once you’ve located the ECU and removed the cover, you’ll need to identify the diagnostic mode selector switch. This switch is crucial for initiating the ECU’s self-diagnostic functions and retrieving error codes. The type of switch varies slightly depending on the engine type and specific ECU model:

VG30i ECU: These ECUs, often found in V6 models of this era, are equipped with a tiny toggle switch on the side of the ECU. This toggle switch has a small, snap-in cover that needs to be removed to access the switch.

Diagnostic selector switch with cover installed, showcasing the location on the ECU.

Diagnostic selector switch with the cover removed, revealing the toggle switch.

4-Cylinder ECU & VG30E ECU: These ECUs, used in 4-cylinder models and some V6 engines (VG30E), typically feature a slotted switch. This switch is rotated using a screwdriver, usually a quarter or half turn clockwise and counterclockwise, to select the diagnostic mode. Some early 4-cylinder Hardbody (HB) ECUs might also have the toggle switch.

Regardless of whether you have a toggle or slotted switch, the function remains the same: to engage the ECU’s diagnostic mode.

Retrieving Error Codes: Procedure for 4-Cylinder Engines

Follow these steps to retrieve error codes from 4-cylinder Nissan engines of this era:

Ignition ON (Engine OFF): Turn the ignition key to the “ON” position, but do not start the engine. All the dashboard indicator lights should illuminate.
Turn Diagnostic Mode Selector Clockwise: Locate the slotted ‘diagnostic mode selector’ screw on the ECU. Turn it fully clockwise. The check engine light on your dashboard should turn off.
Wait and Turn Counter-Clockwise: Wait for 5 seconds, and then turn the slotted screw fully counter-clockwise.
Observe Check Engine Light Flashes: The check engine light will now begin to flash a sequence of codes. These codes are communicated through flashes:
- Long flashes: Represent the first digit of the error code (tens place).
- Short flashes: Represent the second digit of the error code (ones place).
- For example, two long flashes followed by three short flashes indicate error code 23.
Clear Stored Codes (Optional): To clear any stored error codes and turn off the check engine light, turn the slotted screw fully clockwise while the ECU is in diagnostic mode (check engine light flashing). Wait 5 seconds, and then turn the screw fully counter-clockwise.
Keep Codes Stored (Optional): If you want to keep the error codes stored in the ECU’s memory, simply turn the ignition key to the “OFF” position while the check engine light is flashing. The ECU will retain the stored codes, and the check engine light will remain illuminated when you next start the vehicle (if the fault is still present).

Retrieving Error Codes: Procedure for V6 Engines (VG30i & VG30E)

The process for retrieving error codes from V6 engines is slightly different, depending on whether you have a VG30i or VG30E engine.

For VG30i Engines (Toggle Switch):

Ignition ON (Engine OFF): Turn the ignition key to the “ON” position, without starting the engine.
Access Diagnostic Switch: Locate the small plastic tab on the side of the ECU (facing the door). Pull this tab off to reveal the toggle switch.
Set Switch to “DIAGNOSTICS”: Using a small screwdriver or tool, 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 is being selected.
Mode Selection Blinking: The LEDs will blink to indicate the diagnostic mode number. They will blink once, pause, then twice, pause, three times, pause, and so on, cycling through modes 1 through 5. Mode 3 is the most commonly used mode for retrieving ECU trouble codes.
Select Mode 3: After the LEDs blink three times (indicating Mode 3), move the toggle switch back to the “ON” position.
Observe LED Flashes for Error Codes: After about five seconds, the ECU will start outputting diagnostic codes. These codes are displayed using the red and green LEDs:
- Red LED flashes: Indicate the tens digit of the error code.
- Green LED flashes: Indicate the ones digit of the error code.
- For example, one red flash followed by two green flashes represents error code 12.
Clear ECU Codes (Optional): To clear trouble codes, move the switch to the “DIAGNOSTICS” position and then immediately back to the “ON” position. Then, turn the ignition key to the “OFF” position.

For VG30E Engines (Slotted Switch):

The procedure for VG30E engines with a slotted switch is identical to the 4-cylinder engine procedure outlined above. Refer to the “Retrieving Error Codes: Procedure for 4-Cylinder Engines” section.

Understanding Nissan OBD1 Error Codes (1986.5-1995)

Once you have retrieved the error codes by observing the flashes, you’ll need to interpret them. Here’s a comprehensive list of Nissan ECU flash codes for models from 1986.5 to 1995, applicable to various engines including those in the 1987 Nissan 200SX, Pathfinder, and Hardbody pickups:

11: Crank Angle Sensor/Camshaft Position Sensor.
12: Air Flow Meter/Mass Air Flow Sensor.
13: Engine Coolant Temperature Sensor.
14: Vehicle Speed Sensor.
21: Ignition Signal.
22: Fuel Pump.
23: Idle Switch.
24: Throttle Valve Switch.
25: Idle Speed Control Valve.
28: Cooling Fan Circuit.
31: ECM (ECU).
32: EGR Function.
33: Heated Oxygen Sensor.
34: Knock Sensor.
35: Exhaust Gas Temperature Sensor.
36: EGR Control-Back Pressure Transducer.
37: Knock Sensor (likely duplicate, may refer to different knock sensor or condition).
38: Right hand bank Closed Loop (B2).
41: Intake Air Temperature Sensor.
42: Fuel Temperature Sensor.
43: Throttle Position Sensor.
44: ECCS Normal Operation (No Malfunction).
45: Injector Leak.
47: Crankshaft Position Sensor (likely duplicate, may refer to different condition).
51: Injector Circuit.
53: Oxygen Sensor (likely duplicate, may refer to different sensor or condition).
54: A/T Control (Automatic Transmission Control).
55: No Malfunction (No Error Codes).
63: No. 6 Cylinder Misfire.
64: No. 5 Cylinder Misfire.
65: No. 4 Cylinder Misfire.
66: No. 3 Cylinder Misfire.
67: No. 2 Cylinder Misfire.
68: No. 1 Cylinder Misfire.
71: Random Misfire.
72: TWC Function right hand bank (Three-Way Catalytic Converter).
73: TWC Function right hand bank (Three-Way Catalytic Converter) (likely duplicate, may refer to different condition).
76: Fuel Injection System Function right hand bank.
77: Rear Heated Oxygen Sensor Circuit.
82: Crankshaft Position Sensor (likely duplicate, may refer to different condition).
84: A/T Diagnosis Communication Line (Automatic Transmission Diagnosis).
85: VTC Solenoid Valve Circuit (Variable Timing Control).
86: Fuel Injection System Function right hand bank (likely duplicate, may refer to different condition).
87: Canister Control Solenoid Valve Circuit.
91: Front Heated Oxygen Sensor Heater Circuit right hand bank.
94: TCC Solenoid Valve (Torque Converter Clutch).
95: Crankshaft Position Sensor (likely duplicate, may refer to different condition).
98: Engine Coolant Temperature Sensor (likely duplicate, may refer to different condition).
101: Front Heated Oxygen Sensor Heater Circuit right hand bank (likely duplicate, may refer to different condition).
103: Park/Neutral Position Switch Circuit.
105: EGR and EGR Canister Control Solenoid Valve Circuit.
108: Canister Purge Control Valve Circuit.

This comprehensive list allows you to diagnose a wide range of potential issues indicated by the ECU in your 1987 Nissan 200SX or similar Nissan vehicle.

Exploring ECU Diagnostic Modes (Mode I – Mode V)

Beyond retrieving stored fault codes, the Nissan OBD1 ECU offers several diagnostic modes designed to monitor specific system functions in real-time. These modes can be accessed by cycling through the diagnostic selector switch or toggle switch. Here’s a breakdown of each mode:

Mode I – Exhaust Oxygen Sensor Monitor:

Purpose: To check the functionality of the Oxygen Sensor.
Procedure:
1. Warm the engine to its normal operating temperature.
2. Run the engine above 2000 RPM with no load (e.g., in neutral or park).
3. Observe the GREEN LED on the ECU.
Interpretation:
- Functional Sensor: The GREEN LED should blink ON and OFF more than five times within ten seconds at 2000 RPM.
- Suspect Sensor: If the LED flashes fewer than five times, the oxygen sensor may need replacement.
- Circuit Issue: If the LED does not flash at all, check the oxygen sensor’s wiring and circuit for faults.

Mode II – Mixture Ratio Control Monitor:

Purpose: To assess if the air/fuel mixture is cycling correctly (indicating proper closed-loop fuel control).
Procedure:
1. Warm the engine to normal operating temperature.
2. Engage Mode II: Turn the diagnostic selector dial fully clockwise (or toggle switch to “DIAGNOSTICS”). After the LEDs flash twice, turn the dial fully counter-clockwise (or toggle switch back to “ON”).
3. Run the engine above 2000 RPM with no load.
4. Observe the RED and GREEN LEDs.
Interpretation:
- Correct Cycling: The RED LED should blink simultaneously with the GREEN LED, indicating proper air/fuel mixture cycling.
- Rich Mixture: If the RED LED stays OFF, the mixture is likely too rich (more than 5% rich).
- Lean Mixture: If the RED LED stays ON, the mixture is likely too lean (more than 5% lean).
- Persistent Lean/Rich: If the RED LED remains ON or OFF even above 2000 RPM, complete the full diagnostic code retrieval (Mode III) before proceeding with repairs.

Mode III – Stored Fault Code Reporting:

Purpose: To retrieve all stored trouble codes from the ECU’s memory.
Procedure: This is the primary mode for retrieving error codes, detailed in the “Retrieving Error Codes” sections above for both 4-cylinder and V6 engines. Use the procedures outlined earlier in this guide to access Mode III and read the fault codes.

Mode IV – Switch Operation Monitor:

Purpose: To verify the proper operation of the Throttle Position Sensor (TPS), Starter circuit, and Vehicle Speed Sensor (VSS).
Procedure:
1. Ignition ON (Engine OFF).
2. Engage Mode IV: Turn the diagnostic selector dial fully clockwise. After the LEDs flash four times, turn the dial fully counter-clockwise.
3. TPS Test: Ensure the RED LED is initially OFF. Depress the gas pedal fully. The RED LED should turn ON when the pedal is depressed. If not, inspect or replace the Throttle Position Sensor.
4. Starter Circuit Test: Turn the ignition key to the “START” position (momentarily engage the starter). The RED LED should turn ON while the key is in the “START” position. If not, check or repair the starter circuit.
5. VSS Test: With the ECU cover removed and ECU visible, drive the vehicle at a speed of 12 MPH or more. The GREEN LED should turn ON when the vehicle speed exceeds 12 MPH. If not, check or replace the Vehicle Speed Sensor.

Mode V – Real Time Diagnostic Function:

Purpose: To diagnose issues with the Crankshaft Position Sensor (CPS), Mass Air Flow Sensor (MAFS), and Ignition signal while the engine is running.
Procedure:
1. Start the engine.
2. Engage Mode V: Turn the diagnostic selector dial fully clockwise. After the LEDs flash five times, turn the dial fully counter-clockwise.
3. Monitor LEDs: Observe the LEDs for flashes while the engine is idling and when revved (“racing” the engine).
Interpretation:
- No Flashes (Normal): Ideally, the LEDs should not flash for 5 minutes during idling or revving if these systems are functioning correctly. If no flashes occur, turn off the engine.
- RED LED Flashes 3 Times (1.5 seconds): Indicates a potential issue with the Crankshaft Position Sensor. Check the CPS harness for continuity, inspect for dust or contamination, and check ECU pin terminals. Tap the harness connector or CPS during real-time diagnosis to see if the code reappears, which could indicate a faulty terminal.
- GREEN LED Flashes 2-3 Times (0.5 seconds): Suggests a possible problem with the Mass Air Flow Sensor. Perform similar harness and terminal checks as described for the CPS.
- GREEN LED Flashes 4 Times (0.2 seconds): Points to a potential issue with the Ignition signal. Again, check harness continuity and terminals as described previously.

Location of the diagnostic LEDs on the ECU, used for displaying error codes and diagnostic mode feedback.

Conclusion: DIY OBD1 Diagnostics for Your Nissan Classic

Diagnosing your 1987 Nissan 200SX or similar OBD1-equipped Nissan vehicle doesn’t require expensive, specialized 1987 Nissan 200SX OBD1 scan tools. By understanding the location of your ECU, identifying the diagnostic switch, and following these step-by-step procedures, you can effectively retrieve error codes and utilize the ECU’s built-in diagnostic modes. This empowers you to troubleshoot common issues, understand your vehicle’s needs, and take proactive steps in maintaining your classic Nissan for years to come. Remember to consult your vehicle’s service manual for detailed repair procedures once you’ve identified the potential problem areas using these diagnostic methods.