The check engine light – that small, often amber or yellow icon on your dashboard – can be a source of anxiety for any car owner. When it illuminates, it’s your car’s way of saying something isn’t quite right under the hood. While modern vehicles often require specialized tools for diagnosis, older Subaru models offer a unique, do-it-yourself approach to understanding those cryptic engine codes, sometimes referred to in online communities in relation to blue wires used in the diagnostic process. This guide will walk you through how to diagnose your Subaru’s check engine light using this manual method, allowing you to potentially pinpoint the issue before heading to a mechanic.
Understanding the Subaru Check Engine Light System
Before diving into the diagnostic procedure, it’s crucial to understand what the check engine light signifies. This light is part of your car’s onboard diagnostic system (OBD), specifically in older Subarus, often an earlier, pre-OBD-II system. When the engine control unit (ECU) detects a problem in the engine, emissions, or related systems, it triggers the check engine light to alert the driver.
While a lit check engine light can indicate a serious problem, it can also be triggered by minor issues like a loose gas cap. Therefore, diagnosing the specific code is essential to understanding the severity and nature of the problem.
Manually Reading Subaru Check Engine Codes: The Blue Wire Method
For certain older Subaru models, you can retrieve diagnostic trouble codes (DTCs) without a scan tool. This method involves locating specific blue wires and a black connector within the vehicle. Here’s a step-by-step guide:
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Locate the Diagnostic Connector: Begin by removing the kick panel on the driver’s side, near the footrest. Behind this panel, you’ll find a black diagnostic connector and typically, a set of blue wires.
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Prepare the Connector: Identify the black connector. It has multiple pins and a notch on one side. Hold the connector with the notch facing upwards.
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Connect the Blue Wire for Code Reading: With the ignition turned off, take one of the blue wires and insert it into the top row of the black connector, specifically into the second pin from the left.
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Initiate Diagnostic Mode: Turn the ignition key to the “ON” position. Do not start the engine.
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Observe the Check Engine Light: Now, watch the check engine light on your dashboard. It will begin to blink in a pattern. These blinks are how the system communicates the diagnostic codes.
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Count the Blinks: Carefully count the blinks. Long blinks represent ’10’, and short blinks represent ‘1’. For example, two long blinks followed by three short blinks would indicate code 23. You’ll need to observe the blinking pattern until the code sequence repeats to ensure accurate reading. It might feel mesmerizing as you focus on the blinking light!
Clearing Check Engine Codes Manually
After reading the codes and addressing the issue, you might want to clear the codes and turn off the check engine light. Here’s how to clear codes using the blue wire method:
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Prepare for Code Clearing: Ensure the engine is off. This time, take the blue wires and insert them into the first and second pin in the top row of the black connector, again counting from the left with the notch facing upwards.
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Start and Stop Engine: Start the car briefly, then immediately shut it off.
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Disconnect Wires: Remove the blue wires from the connector.
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Verify Codes are Cleared: Turn the ignition back to the “ON” position (engine off) and read the codes again as described in the “Reading Codes” section. If the codes are cleared, the check engine light will blink steadily at approximately ¼ second intervals, indicating no stored codes.
Subaru OBD-I Diagnostic Trouble Codes
Once you’ve retrieved the blink codes, you’ll need to interpret them. Here is a list of common OBD-I codes for older Subaru models, as referenced in the original forum post:
| Code | Probable Cause |
|---|---|
| 11 | Crank Angle Sensor or Circuit |
| 12 | Starter Switch or Circuit |
| 13 | Cam Position Sensor or Circuit (TDC Sensor on Justy) |
| 14 | Fuel Injector No. 1 (Various Models) / Injectors 1 & 2 (XT, Loyale, GL, DL) |
| 15 | Fuel Injector No. 2 (Various Models) / Injectors 3 & 4 (Loyale, GL, DL) / Injectors 5 & 6 (XT-6) |
| 16 | Fuel Injector No. 3 (Various Models) / Injectors 3 & 4 (XT) |
| 17 | Fuel Injector No. 4 (Various Models) / Injectors 1 & 2 (XT-6) |
| 18 | Fuel Injector No. 5 (SVX) |
| 19 | Fuel Injector No. 6 (SVX) |
| 21 | Coolant Temperature Sensor or Circuit |
| 22 | Knock Sensor or Circuit (Right Side on SVX) |
| 23 | Air Flow Meter or Circuit (Exc. Justy) / Pressure Sensor (Justy) |
| 24 | Air Control Valve or Circuit (Exc. Justy) / Idle Speed Control Solenoid Valve (Justy) |
| 25 | Fuel injector No. 3 and 4 (XT-6) |
| 26 | Air Temperature Sensor (Justy) |
| 28 | Knock Sensor No. 2 (SVX, Left Side) |
| 29 | Crank Angle Sensor (SVX, No. 2) |
| 31 | Throttle Position Sensor or Circuit |
| 32 | Oxygen Sensor or Circuit (No. 1, Right Side, On SVX) |
| 33 | Vehicle Speed Sensor (VSS) or Circuit |
| 34 | EGR Solenoid or Circuit |
| 35 | Purge Control Solenoid or Circuit |
| 36 | Air Suction Solenoid Valve (Impreza) / Ignitor Circuit (Justy) |
| 37 | Oxygen Sensor (No. 2, Left Side, On SVX) |
| 38 | Engine Torque Control (SVX) |
| 41 | Air/Fuel Adaptive Control |
Important Notes:
- Oxygen Sensors (O2 Sensors) and Code 32/37: The original forum post mentions that a Blue Check Engine Light usually indicates the need to replace O2 sensors. Codes 32 and 37 specifically point to oxygen sensor issues. However, always verify the code and consider other potential causes.
- Model Specifics: Note that some codes and their meanings can vary slightly depending on the exact Subaru model and year. Always consult your vehicle’s repair manual for the most accurate information.
- Limitations of OBD-I: This manual method is applicable to older Subaru vehicles, likely those preceding the widespread adoption of OBD-II systems. Modern cars typically require an OBD-II scanner for code retrieval.
- Seek Professional Help: If you are uncomfortable performing these steps, or if you are unsure about the meaning of the codes or how to address the underlying issue, it’s always best to consult a qualified mechanic. Incorrect diagnosis or repair attempts can potentially cause further damage to your vehicle.
While the “blue check engine light” might be a misnomer, referring to the diagnostic blue wires used in the process, understanding how to manually retrieve and interpret codes on older Subarus can be a valuable skill for DIY enthusiasts and owners of these reliable vehicles.
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