P0645Generic code

P0645 Code: A/C Clutch Relay Control Circuit

The P0645 code means the computer found an electrical fault in the control circuit of the A/C compressor clutch relay. It can leave you with an air conditioner that does not cool. Check the fuse, relay, connector and wiring first.

Severity
Low
Can I keep driving?
Yes, no immediate risk
Estimated cost
$15 to $300
Difficulty
Medium
System
Computer and Outputs
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0645 code means

The computer engages the electromagnetic clutch on the A/C compressor through a relay, after it checks the refrigerant pressure and the evaporator temperature.

The computer watches the control circuit from its own driver transistor. The P0645 code flags an electrical fault in that control circuit. Before you replace the component, check the fuse, relay, connector and wiring, which are the most common causes.

If the relay does not close, the compressor does not engage and the air conditioner does not cool. A stuck relay keeps the compressor engaged all the time, which can freeze up the evaporator. Swapping the relay with an identical one is the quickest test.

Always start with the simplest checks: fuse, relay and connector. Then check power, ground and signal with the component connected (using back-probes) and, last, the continuity of each wire to the computer. Replace the component only when all of that checks out.

P0645 code symptoms

  • Check engine light or another warning light on.
  • The air conditioner does not cool.
  • The compressor does not engage (you do not hear the click).

Likely causes of the P0645 code

  1. Damaged connector. Loose, broken or corroded.
  2. Blown or defective fuse or relay.
  3. Wiring cut or shorted between the component and the computer.
  4. Failed component. The coil, the motor or the internal electronics.

How to diagnose the P0645 code step by step

  1. Activate the component with the scan tool and check whether it responds.
  2. Check the fuse and relay. Measure the voltage on both sides of the fuse, and swap the relay with an identical one.
  3. Measure the supply at the connector. You should see battery voltage with the key on or when the component is activated.
  4. Measure the resistance of the component and compare it with the manual. Also check the insulation to ground.
  5. Check the control wire for continuity to the computer and for shorts.
  6. Check the compressor clutch air gap if the relay works but the clutch does not engage.

How to fix the P0645 code

  • Repair the connector, the wiring or the power supply.
  • Replace the A/C clutch relay if it does not respond even with correct power and command signals.
  • Clear the codes and verify the repair.

Technical reference data

  • 12 volt relay. The compressor clutch coil measures 3 to 5 ohms.
  • Reference values: check the manufacturer data for your vehicle.

Brand-specific notes

This code is very frequent on a few makes, and the relay swap test is worth doing on all of them.

P0645 on Nissan

Very frequent on Nissan.

P0645 on Toyota

Very frequent on Toyota.

P0645 on Chevrolet

Very frequent on Chevrolet.

Frequently asked questions about P0645

Can I drive with a P0645 code?
Yes, with caution. The engine runs, but the air conditioner will not cool, and a stuck relay could keep the compressor running all the time.
What does the A/C clutch relay do?
The computer uses it to engage the compressor clutch, after checking the refrigerant pressure and the evaporator temperature.
Do I always have to replace the part?
No. Check the fuses, relays, connectors and wiring first, because they are the most common cause.
How much does it cost to fix P0645?
As a rough guide, between $15 and $300. A relay or fuse is at the low end.
What does the P0645 code mean?
It is a generic OBD2 code that means the computer found an electrical fault in the A/C compressor clutch relay control circuit.
What is the most common cause of P0645?
A faulty relay, a blown fuse or a damaged connector. Swapping the relay with an identical one is the quickest way to check a P0645 code.

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Questions about the P0645 code

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