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DTC codes / Volkswagen codes / P1657

P1657 · VolkswagenA/C Clutch Relays Circuit Short To B+

P1657 on Volkswagen vehicles means the engine control module (ECM) has detected a short to battery positive voltage (B+) in the air conditioning (A/C) clutch relay control circuit. The ECM expects to control the relay by providing a ground path, but it sees constant voltage on the control wire, indicating a wiring fault or a failed relay.

SystemPowertrain
Code typeVolkswagen specific
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • The A/C compressor clutch may not engage, resulting in no cold air from the vents.
  • The A/C clutch may stay engaged continuously, even with the A/C switched off, causing the compressor to run unnecessarily.
  • The check engine light (malfunction indicator lamp) illuminates on the dashboard.
  • In some cases, the A/C system may blow warm air or fluctuate between cold and warm.
  • Often no symptoms besides the warning light, especially if the short is intermittent or only affects the control circuit without immediate drivability impact.

Common causes, most likely first

  1. Faulty A/C clutch relay, internally shorted to battery voltage.
  2. Chafed or damaged wire in the A/C clutch relay control circuit shorting to a constant power source.
  3. Corroded or loose connector at the A/C clutch relay or ECM, causing unintended voltage feedback.
  4. Failed ECM driver for the A/C clutch relay, though rare.
  5. Aftermarket accessories or improper wiring repairs tapping into the A/C circuit.
  6. Water intrusion into the relay box or wiring harness, leading to corrosion and shorts.

How to diagnose and fix it

  1. Verify the fault by scanning all modules and checking freeze-frame data for the A/C clutch relay control circuit status.
  2. Inspect the A/C clutch relay and its socket for corrosion, loose terminals, or signs of overheating; swap with a known-good relay of the same part number and recheck.
  3. Disconnect the A/C clutch relay and measure voltage on the control circuit at the relay socket with the key on; if battery voltage is present with the relay removed, trace the wire for a short to B+.
  4. Visually inspect the wiring harness from the relay to the ECM for chafing, pinches, or damage near sharp edges or heat sources.
  5. Perform a wiggle test on the harness while monitoring the circuit with a multimeter or scan tool to identify intermittent shorts.
  6. If the wiring is intact, test the ECM control circuit with a test light or oscilloscope to confirm the ECM is commanding the relay correctly; replace the ECM only if all other possibilities are eliminated.
  7. Clear the fault codes and road-test the vehicle with the A/C on and off to confirm the repair.

Before you replace the A/C clutch relay

  • A short to B+ in the control circuit can be caused by a failed relay that internally bridges the power and control terminals, so always test the relay before condemning the wiring.
  • The ECM may set P1657 even if the A/C system appears to work normally, because the short can be intermittent and only detected under specific voltage conditions.
  • Aftermarket alarm or remote start installations often tap into the A/C clutch circuit, creating a short to power that triggers this code.
  • On some Volkswagen models, the A/C clutch relay is located in the engine bay fuse box, and water intrusion from a clogged drain can corrode the relay terminals and cause a short.
  • Do not confuse this code with P1658 or P1659, which refer to different A/C clutch circuit faults; P1657 specifically indicates a short to B+ on the control circuit.

Can I keep driving with P1657?

You can generally drive the vehicle with this code, but avoid using the A/C until the fault is repaired, as a shorted relay could cause the compressor to run continuously and lead to overheating or damage. If the compressor stays engaged with the A/C off, have the system inspected promptly.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Volkswagen vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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