What Does This Indicator Mean?
The Subaru Yellow Glow Plug Warning Light, which typically resembles a set of coiled pig-tail loops, is a dedicated dashboard indicator found exclusively on diesel-powered Subaru models utilizing the Boxer Diesel (EE20) engine architecture. Unlike standard gasoline ignition systems that rely on high-voltage spark plugs, diesel engines utilize compression ignition. Glow plugs are high-output electric heating elements threaded directly into each cylinder’s combustion chamber to pre-heat the air-fuel mixture during cold ambient starts. When this yellow indicator illuminates steadily or flashes upon cycling the ignition, it signifies that the Engine Control Module (ECM) or Glow Plug Control Unit has detected a fault within the pre-heating or post-heating electrical circuits, such as excessive electrical resistance, a compromised glow plug relay, or a thermal failure within one or more individual glow plug elements.
Can I Still Drive with This Light On?
Because this warning carries a yellow severity rating, the mechanical verdict depends heavily on when the light illuminates and whether it is accompanied by a flashing check engine light or limp mode. If the light comes on only during a cold start and goes out shortly after, you can generally drive the vehicle directly to a repair facility, though you may experience extended cranking times, rough idling, and elevated white exhaust smoke until the engine reaches operating temperature. However, if the glow plug light flashes continuously while driving, it indicates a critical powertrain fault detected by the ECM, often tied to the Diesel Particulate Filter (DPF) regeneration cycle or fuel injection system. Continued operation in this state can lead to severe catalytic and DPF fouling, fuel dilution of the engine oil, and permanent internal engine component degradation.
Common Causes of the Warning
- Burnt-Out or Degraded Glow Plug Elements: Over thousands of thermal cycles, the internal resistance of the metallic heating coil inside the glow plug increases, eventually fracturing or shorting out completely, which triggers an open-circuit fault code in the ECM.
- Faulty Glow Plug Control Module or Relay: The solid-state relay responsible for modulating high-amperage electrical current to the glow plugs can experience internal contact pitting, thermal fatigue, or circuit board failure, cutting power to the heating elements entirely.
- High Resistance in Circuit Wiring or Connectors: Corrosion, thermal degradation, or oil contamination within the main glow plug harness connectors can introduce excessive electrical resistance, preventing the ECM from delivering the precise current required for proper pre-heating.
How to Diagnose and Reset It
Diagnosing a Subaru glow plug fault requires a systematic approach using both electronic diagnostic tools and basic hardware inspection. First, connect an advanced OBD-II diagnostic scanner capable of reading manufacturer-specific diesel powertrain trouble codes (such as P0670 through P0674) to pinpoint which specific cylinder circuit has failed. Next, pop the hood and use a digital multimeter set to ohms to measure the electrical resistance of each individual glow plug; a healthy glow plug typically reads less than two ohms, whereas a dead plug will show infinite resistance or an open circuit. Inspect the wiring harness and connectors for any signs of fraying, melting, or corrosion. Once you replace the faulty glow plug, relay, or harness, the warning light can be cleared by using the OBD-II scanner to command a diagnostic trouble code reset, though self-clearing may occur automatically after several successful driving cycles if the underlying fault has been entirely resolved.