P0645 on 2013-2016 Lincoln MKZ: A/C Clutch Relay Circuit Causes and Fixes

What's Unique About the 2013-2016 Lincoln MKZ

For the 2013-2016 Lincoln MKZ, which is built on Ford's global CD4 platform, this code is a common and straightforward electrical issue. Its platform-mate, the Ford Fusion, frequently experiences the exact same P0645 problem with the same diagnostic path. Lincoln's parent company, Ford, issued a Special Service Message (SSM-45815) for 2011-2017 vehicles, acknowledging that P0645 can cause an inoperative A/C system and advising technicians that if the A/C clutch can be commanded on with a scan tool, replacing the PCM will not fix the issue. This confirms the problem is a known issue and not an anomaly.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Air conditioning does not blow cold air or is completely inoperative.
• Check Engine Light is on.
• A/C system may work intermittently.
• A clicking or buzzing noise may be heard from the A/C relay as it tries to engage.
• The A/C indicator light on the climate control panel might blink.

Most Likely Causes

1. Faulty A/C Clutch Relay 🔴 High Probability → Shop Wiring Relay Relays are electromechanical switches that wear out from repeated on/off cycles. Given the shared platform with Ford Fusion, this specific relay is a very common failure point for A/C issues.
   How to confirm: Locate the A/C clutch relay (marked as R7) in the under-hood fuse box, which is next to the air filter. 🎬 Watch: Locate your A/C clutch relay in the engine bay. Swap it with an identical relay from a non-critical system, such as the horn relay. If the A/C starts working and the horn stops, the relay is bad. This is the most common and effective first diagnostic step.
   Typical fix: Replace the faulty relay with a new one. The OEM part number is often superseded, but a common replacement is Motorcraft HU5Z-14N089-B.
   Est. part cost: $15-$40
2. Wiring or Connector Issue 🟡 Medium Probability Engine bay wiring is exposed to heat, vibration, and moisture, which can lead to chafed insulation, corrosion on connectors, or broken wires over time.
   How to confirm: Visually inspect the wiring harness leading to the A/C compressor and the A/C relay socket for any signs of damage, corrosion, or loose connections. Use a multimeter to check for continuity and proper voltage at the relay socket terminals.
   Typical fix: Repair the damaged section of wire or clean/replace the corroded connector.
   Est. part cost: $5-$50
3. Blown Fuse ⚪ Low Probability
   How to confirm: Check fuse F28 (a 10A fuse) in the under-hood fuse box, which protects the A/C clutch relay circuit. A visual inspection or a multimeter test can confirm if it's blown. A blown fuse often indicates an underlying short circuit that caused it to blow.
   Typical fix: Replace the blown fuse. If it blows again, a short circuit in the wiring or the A/C clutch coil must be diagnosed and repaired.
   Est. part cost: $1-$5
4. Failed A/C Compressor Clutch Coil ⚪ Low Probability → Shop A/C Compressor
   How to confirm: With the engine off, disconnect the electrical connector at the A/C compressor. Use a multimeter to measure the resistance of the clutch coil. A reading that is outside the manufacturer's specification (typically between 3.0 and 5.0 ohms) or shows an open circuit (infinite resistance) indicates a failed coil. Readings below 2.0 ohms or above 5.0 ohms are generally considered unacceptable.
   Typical fix: On many modern compressors, the clutch is not sold separately, so the entire A/C compressor assembly must be replaced. This requires professional evacuation and recharging of the refrigerant. However, for some MKZ models (specifically 3.7L), the coil may be available separately.
   Est. part cost: $250-$600

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): This is the least likely cause. As noted in TSB SSM-45815, if the A/C can be manually activated with a diagnostic tool, the PCM is not the problem. All other possibilities, including wiring, the relay, and the clutch itself, should be exhaustively tested before condemning the PCM.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner and confirm P0645 is present.
2. Visually inspect the A/C compressor, its wiring connector, and the harness for any obvious signs of damage.
3. Locate the under-hood fuse box. Check fuse F28 (10A) for the A/C clutch.
4. Locate the A/C clutch relay in position R7 of the under-hood fuse box.
5. Swap the A/C clutch relay (R7) with a known-good, identical relay (e.g., horn relay) to quickly test if the relay is the fault. If the A/C now works, the relay was bad.
6. If the relay is not the issue, use a multimeter to check for battery voltage at the power-feed terminal of the relay socket.
7. With the relay removed, have a helper turn the A/C on. Check for a ground signal from the PCM on the control side of the relay socket.
8. If power and ground signals are correct at the relay socket, check the wiring continuity from the relay to the A/C compressor clutch.
9. If wiring is good, test the resistance of the A/C compressor clutch coil. It should be between 3.0 and 5.0 ohms.
10. If all components and wiring test good, the fault may lie within the PCM, which requires advanced diagnostics as outlined in TSB SSM-45815.

Parts You'll Likely Need

• A/C Clutch Relay (OEM #HU5Z-14N089-B (supersedes 5L3Z-14N089-AA, 8T2Z-14N089-C)) — This is the most common failure point for a P0645 code and is the easiest and cheapest part to replace for diagnosis.
  Trusted brands: Motorcraft, Bosch, Standard Motor Products
  OEM price range: $25-$50
  Aftermarket price range: $15-$40
• A/C Compressor with Clutch (OEM #EG9Z-19703-B (for 2.0L w/ R134A, example)) — If the A/C compressor's clutch coil has failed, the entire compressor assembly is typically replaced. This is a more involved and expensive repair.
  Trusted brands: Motorcraft, Denso, Four Seasons
  OEM price range: $400-$700
  Aftermarket price range: $250-$600

Technical Service Bulletins (TSBs) & Recalls

• SSM-45815: AIR CONDITIONING (A/C) INOPERATIVE - DIAGNOSTIC TROUBLE CODE (DTC) P0645

Platform-Specific Known Issues

• A Technical Service Bulletin (SSM-45815) was issued for 2011-2017 Ford/Lincoln vehicles, explicitly linking DTC P0645 to an inoperative A/C system and providing diagnostic guidance to avoid unnecessary PCM replacement.
• The under-hood fuse box contains both fuses and relays on top, with more fuses on the bottom, requiring the box to be unlatched and pivoted for full access.
🎬 See how to access the under-hood power distribution box.

Mechanic-Grade Diagnostic Values

• A/C Clutch Relay Socket Voltage (Power Feed) — expected: Greater than 10 Volts. Failure: Voltage less than 10V indicates an open or high resistance in the power supply circuit (CBB40).
• A/C Clutch Relay Control Circuit Voltage (at PCM) — expected: Approx. 0 Volts when A/C is commanded ON. Failure: Voltage stays high when A/C is commanded on, indicating the PCM is not providing ground.
• Ground Strap/Connection Resistance — expected: Near 0 ohms. Failure: Any significant resistance indicates a poor ground connection, which can cause intermittent issues.

Scan Tool Commands That Help

• FORScan: ACC_CMD (A/C Clutch Command) — Use this bidirectional control to manually command the A/C compressor clutch on and off. This is the definitive test to determine if the fault is on the control side (PCM, wiring) or the component side (clutch), as mentioned in TSB SSM-45815.

Wiring & Ground Locations

• Circuit CH302 (WH/BN) — Wire between the A/C clutch relay socket and the PCM connector C175b, pin 14.. This is the specific control wire that the PCM grounds to activate the A/C relay. A break or short in this wire is a direct cause of P0645.
• Circuit CBB40 (YE/GN) — Wire from fuse F28 in the under-hood fuse box to pin 2 of the A/C clutch relay socket.. This wire supplies the power to the relay's control coil. A lack of voltage here will prevent the relay from ever activating.
• G101 — Ground point located on the left front of the engine compartment.. While not exclusively for the A/C clutch, this is a primary engine bay ground point. A loose or corroded G101 can cause numerous intermittent electrical issues, including faults in control module circuits like P0645.

Real Owner Repair Stories

• Reddit user on r/Ford (2016 Ford Explorer (shares electrical architecture with MKZ)) — P0645 code present, A/C not blowing cold, clutch not engaging.
  ❌ Tried (didn't work) Swapping the A/C clutch relay with the identical horn relay did not immediately fix the problem.
  ✅ What actually fixed it The A/C clutch engaged only after the user honked the horn (activating the swapped relay in its new position). This strongly implies the root cause was not the relay itself, but a poor or corroded pin connection in the fuse box socket. The user noted they had spilled gear oil in the fuse box area a year prior, likely causing the connection issue.

OEM Part Supersession History

• 5L3Z-14N089-AA → 8T2Z-14N089-C, then current part HU5Z-14N089-B — Standard part revision and consolidation by the manufacturer.
  Heads up: All versions are considered interchangeable for this application.

Model Year Variations Within This Range

• 2013-2016: This range represents the pre-facelift second generation MKZ. It is primarily characterized by the MyLincoln Touch infotainment system and touch-sensitive dash controls, which were replaced in 2017 with Sync 3 and physical buttons. The core powertrain and electrical systems related to code P0645 are consistent across this 2013-2016 range, including for Hybrid models.