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

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

More related articles

How to Renew Your Car Registration in Oklahoma(DELETED)
Having your car registered with the Oklahoma Tax Commission is an important part of driving legally in this state. You will need to do this within 30 days of moving to Oklahoma...
P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
P2422 OBD-II Trouble Code: Evaporative Emissions Control System (EVAP) Vent Valve Stuck Closed
Diagnostic Trouble Code (DTC): P2422 P2422 code definition Evaporative Emissions Control System (EVAP) Vent Valve Stuck Closed Related Trouble Codes: P2441: EVAP Vent Valve Stuck Open EVAP trouble...

Related questions

Q: What would cause brand new O2 sensors to give codes 172, 173, 176, and 177, rich and lean conditions, both sides at the same time?

When it comes to across-the-board codes being set such as this, it is usually a result of something "upstream" creating the condition to set all these codes. For instance, it is rare for both O2 sensors to fail at exactly...

Q: I recently had a new stereo installed in my mirage, and now the airbag light is on. How do i reset it or get it to go out.

The Air Bag Light would set a code in the computer and should be checked by the dealer since you are still under factory warranty unless the stereo installer damaged the wiring or something to do with the airbag. I...

Q: My car will not start

This could be a variety of things ranging from a fuel delivery problem to a no spark problem. The good thing is you know you are getting power to the starter if it cranks. Outside of this you will need...