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DTC codes / P26xx · Computer and output circuits / P2603

P2603Coolant Pump Control Circuit High

P2603 indicates that the engine control module (ECM) has detected an abnormally high voltage on the control circuit for the engine coolant pump, which is typically a pulse-width modulated (PWM) signal used to regulate pump speed. The ECM expects a specific voltage range on this circuit, and a reading above the threshold suggests an open circuit, a short to voltage, or a faulty pump control module.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Engine overheating or elevated coolant temperatures, especially during extended idling or low-speed driving
  • Reduced heater performance or no heat from the cabin vents
  • The coolant pump may run continuously at full speed or not at all, depending on the failure mode
  • In some vehicles, the engine may enter a reduced power mode or set related codes for overheating

Common causes, most likely first

  1. Open or high-resistance wiring in the coolant pump control circuit, such as a broken wire or corroded connector
  2. Short to voltage on the control circuit, often caused by chafed wiring contacting a power source
  3. Faulty coolant pump control module or integrated pump electronics
  4. Failed engine coolant pump itself, particularly if the pump has an internal control circuit
  5. Poor electrical connection at the pump or ECM connector, including loose pins or terminal corrosion
  6. ECM internal fault, though this is less common and should be diagnosed last

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related codes in the engine control system
  2. Visually inspect the coolant pump connector and wiring for damage, corrosion, or loose pins
  3. Measure the control circuit voltage at the pump connector with a digital multimeter and compare to the manufacturer's specification
  4. Perform a continuity and short-to-voltage test on the control circuit between the ECM and the pump
  5. If the circuit tests good, substitute a known-good coolant pump or control module to see if the code clears
  6. Check the ECM connector for corrosion or loose terminals, and repair as needed
  7. Clear the code and perform a road test to confirm the repair, monitoring coolant temperature and pump operation

What the high-voltage code really means

  • A high voltage on the control circuit often indicates an open circuit rather than a short, because the ECM pulls the circuit high when it detects no load.
  • Some vehicles use a three-wire coolant pump with a built-in control module; a failure in the pump electronics can mimic a wiring fault, so always test the pump before replacing the ECM.
  • Corrosion at the pump connector is a frequent cause that is easily overlooked, especially on vehicles driven in wet or salty conditions.
  • The code can be set intermittently by a loose connector that makes good contact most of the time, so wiggle-test the harness while monitoring live data.
  • Replacing the coolant pump without checking the control circuit can lead to a repeat failure if the underlying wiring issue is not addressed.

Can I keep driving with P2603?

It is generally safe to drive short distances to a repair shop, but avoid heavy loads or long trips because the coolant pump may not operate correctly and could cause overheating. If the temperature gauge rises above normal, stop and shut off the engine immediately.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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