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P2603 OBD-II Trouble Code: Coolant Pump Control Circuit High

Check Engine Light

P2603 code definition

When the PCM (powertrain control module) detects a problem in the auxiliary coolant pump control circuit, it will record a P2603 code and the Check Engine Light will illuminate. This code may also be detected by a number of supporting control modules, as well as the PCM.

What the P2603 code means

At lower RPMs, the engine is not able to push as much coolant through its system as may be needed to avoid overheating. Thus, to support the coolant system, the coolant pump operates to electronically circulate coolant throughout the system when the vehicle is idling or operating at low RPMs. When the P2603 code is saved, it indicates that the PCM is getting data that shows the coolant pump circuit is operating outside of the manufacturer’s settings for the vehicle.

What causes the P2603 code?

In most cases, the problem causing the PCM to detect a P2603 code will be a faulty coolant pump control relay. In some other cases, the problem may be:

  • Shorted or open wiring or connectors to the CAN (controller area network) bus harness
  • A broken or loose ground wire or strap
  • A defective CAN bus

What are the symptoms of the P2603 code?

Look for these symptoms if a P2603 code has been stored:

  • Illuminated Check Engine Light
  • Overheating engine
  • Weakened heater function

How does a mechanic diagnose the P2603 code?

First, the mechanic will use an OBD-II scanner to identify the code. Then, they’ll visually inspect the relevant wiring and circuits before testing the CAN bus and its harness. After performing each diagnostic step, they’ll clear the codes, start the engine, and take a test drive to see if the code is stored again. If it does not reappear, they may wait and try to replicate the conditions under which the code was stored in the first place to see if there is an intermittent problem. If the code appears again, they’ll take further steps to determine exactly what is wrong and how to repair it.

Common mistakes when diagnosing the P2603 code

Because it is a communication failure code, it’s important to address the P2603 code before addressing any other codes that the PCM has stored. This is because codes related to engine misfires, exhaust, and other problems can actually be caused by a communication failure code like the P2603 code.

In general, this mistake occurs when technicians and vehicle owners diagnose and attempt to fix codes out of order. Always proceed with diagnosing and addressing codes in the order in which they were stored, and you can avoid this issue.

How serious is the P2603 code?

Driving a vehicle while it’s overheating can do permanent damage to the engine, which can cost significant amounts of money to repair. If the P2603 code is saved, there’s a good chance that your vehicle will begin overheating more and more often, especially at low RPMs, until you get the problem resolved. This can lead to other problems, as well, and can eventually make the vehicle dangerous to drive.

What repairs can fix the P2603 code?

In the majority of instances of a P2603 code being stored, the problem can be resolved by replacing the faulty coolant pump control relay. In other instances, it may be solved by:

  • Repairing or replacing the wiring or connectors for the CAN bus harness
  • Reconnecting or repairing loose or broken ground wires
  • Replacing a defective CAN bus

Additional comments for consideration regarding the P2603 code

While the initial symptoms of a P2603 code may not be significant, especially if you aren’t often caught in traffic and/or you don’t have to keep your car at idle for very long, but this kind of code can result in an avalanche effect with code after code getting detected and stored. Each new code can have more noticeable and detrimental symptoms, so take care of the P2603 code now before it results in much worse repairs down the road.

Need help with a P2603 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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