Code Definition

P3003 — Engine coolant heater relay 1 low output

System: Powertrain  |  Type: Manufacturer-Specific  |  Make: Volkswagen

About P3003

P3003 is a manufacturer-specific powertrain diagnostic trouble code recorded for Volkswagen vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P3003 is stored, the module has detected a signal or supply voltage lower than the expected operating range.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

P3003 is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

The engine control module (ECM) sets this code when it detects that Engine Coolant Heater Relay 1 is providing a lower-than-expected output, which may indicate an open circuit, high resistance, or a failing relay. The engine coolant heater helps warm the coolant during cold starts, improving emissions and efficiency. A malfunction can result in reduced heater function, affecting cold-start performance.

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