A P0646 code in your 2020 Volkswagen Jetta indicates low voltage in the A/C clutch relay control circuit. This code is more specific than P0645, telling us the voltage reading is below the expected range. This typically indicates a short to ground or excessive current draw in the control circuit.
Understanding Low Circuit Voltage
When the ECM drives the A/C relay control circuit, it expects to see specific voltage levels. Low voltage suggests the circuit is completing to ground through an unintended path (short), or that something is drawing the voltage down. This prevents proper relay control and affects A/C operation.
Causes of Low Voltage
A short to ground in the relay control wiring is the primary cause. This can occur from chafed insulation, water intrusion, or physical damage. An internally shorted relay coil draws excessive current and creates low readings. Corroded connections with moisture paths act like partial shorts. The ECM's relay driver may have failed in a shorted state.
Symptoms in Your Jetta
The A/C compressor may not engage properly or at all. The check engine light illuminates with P0646 stored. Fuses may blow if the short is severe enough to cause overcurrent. The A/C system may work intermittently if the short condition varies.
Diagnostic Process
Using VCDS or VW dealer tools, check the A/C relay control circuit status. Disconnect the relay and measure circuit resistance to ground—it should be very high (open). If you see low resistance, trace the wiring for short locations. Check the relay's coil resistance against specifications. Inspect wiring carefully for damage, especially in areas exposed to heat or moisture.