Error Code P0646: What It Means & What To Do?

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Diagnostic trouble code P0646 is technically described as A/C Clutch Relay Control Circuit Low. This code indicates that a fault has been detected in the A/C clutch relay control circuit. The code is due to a defective A/C compressor control relay, or an A/C compressor driver in the PCM. Code P0646 is set by the PCM when it detects a low voltage situation within the air conditioning relay control circuit.

P0646 is a diagnostic trouble code (DTC) for "A/C Clutch Relay Control Circuit Low". This code appears when PCM or other supporting control modules has registered a fault in the A/C clutch relay control circuit. This is a generic diagnostic trouble code (DTC) and applies to many OBD-II vehicles. Trouble code P0646 is associated with the air conditioning (A/C) clutch relay control circuit. The purpose of the A/C clutch relay control circuit is to control the operation of the air conditioning. This is accomplished by providing power to the A/C clutch to engage the A/C compressor.

When the PCM or one of the other supporting control modules has detected improper signals within the A/C clutch relay control electrical circuit, code P0646 will be set.

What causes this problem with the A/C Clutch Relay Control Circuit Low?

• Flawed A/C compressor control relay
• Inoperative A/C Clutch
• Faulty or defective wiring
• Loose or corroded connector
• Broken control module ground strap
• Blown fuses
• Fallacious PCM

1. A/C Clutch Relay - The AC compressor relay is responsible for supplying power to the AC compressor and clutch. Without this relay, the AC compressor will not have power and the AC system will not function. Replace or repair it if you discover it faulty.
2. A/C Compressor Clutch - At the heart of the air conditioning unit of a car is the compressor that pressurizes the refrigerant and pumps it through the A/C system. It derives its power directly from the engine flywheel and, in a similar way to the drive chain of the car, a clutch controls its operation. It should be inspected thoroughly and repaired as required.
3. A/C Compressor - The compressor is the power unit of the air-conditioning system that puts the refrigerant under high pressure before it pumps it into the condenser, where it changes from a gas to a liquid. Check it out thoroughly and if you find it faulty replace it.

Common symptoms of this code are:

• Inoperative A/C system
• Malfunctioned A/C
• Illuminated check engine light

If the code is stored but the check engine light is not illuminated, the code may show up as “pending”.

If you can drive without the comfort of A/C then you can fix this code as per your convenience but keeping any trouble code for long time is not good as it may lead to some other issues aswell. It is always advised to diagnose and fix this code by an experienced technician as this circuit can be very complex and include several relays, fuse-able links and fuses in some circumstances. To fix this code following repairs or replacements may help:

• Replacing the A/C clutch relay
• Replacing blown fuse or fuse-able link (if applicable)
• Cleaning corrosion from connectors
• Repairing or replacing wiring
• Repairing or replacing ground straps
• Replacing a control module
• Replacing the A/C clutch
• Replacing the A/C Compressor
• Flashing or replacing PCM

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The step by step analysis and diagnosis is recommended by a qualified technician using high grade diagnostic tools to check for all the problems and solving them. One should follow the below mentioned steps to diagnose this trouble code:

• Visually inspect all wiring and connectors. Repair or replace all the damaged, disconnected, shorted or corroded wiring, connectors or components.
• If wiring, connectors and all other components are working fine, connect the scanner to the diagnostic port and record all stored codes, and freeze frame data. Clear codes and test drive the vehicle to see if the code returns.
• If the code doesn’t immediately return, there could be an intermittent condition. Intermittent conditions are tough to diagnose and may need to be allowed to return and worsen in order to get an accurate diagnosis.
• Check out if any engine or transmission ground straps, cables or wires that were left loose after repairs were performed.
• Use the digital volt/ohmmeter to check continuity between individual connectors. Compare your findings with manufacturer’s specs and repair any open/shorted circuits as required.

Common Mistakes When Diagnosing the P0646 Code

The most common mistake in addressing this code is replacing the A/C components when the fault is simply in the wiring, ground straps, or other components.