P0645 on 2002-2004 Oldsmobile Alero: A/C Circuit Fault Causing No-Start

What's Unique About the 2002-2004 Oldsmobile ALERO

What's unique and often confusing about the P0645 code on the 2002-2004 Alero and its platform-mate, the Pontiac Grand Am, is its ability to cause a 'crank-no-start' condition. A General Motors Technical Service Bulletin (TSB) confirms that the fuse for the A/C circuit also powers other essential engine functions, likely related to fuel delivery. When the A/C compressor clutch shorts out—a common failure—it blows this shared fuse (specifically, the 10A "A/C BFC" fuse in position #37 of the underhood fuse block), disabling the fuel injector pulse and preventing the engine from starting. This is a symptom most owners would never associate with the A/C system.

Diagnostic Flowchart

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

Symptoms You May Notice

• Engine cranks but does not start.
• No power to the fuel injectors (no injector pulse).
• Air conditioning does not work or blows warm air.
• A/C system works intermittently.
• Check Engine Light is on.
• Blown 10-amp fuse in the underhood fuse box (position #37).

Most Likely Causes

1. Blown A/C BFC Fuse (#37) 🔴 High Probability This fuse is often blown as a secondary failure caused by a shorted A/C compressor clutch coil. Its failure is the direct cause of the no-start condition.
   How to confirm: Visually inspect fuse #37 (labeled A/C BFC, 10A) in the underhood fuse panel. If it is blown, replace it. If it blows again immediately or when the A/C is turned on, a short circuit exists.
   Typical fix: Replace the 10-amp fuse. This is often a temporary fix until the root cause (the short circuit) is found.
   Est. part cost: $1-$5
2. Shorted A/C Compressor Clutch Coil 🔴 High Probability → Shop A/C Compressor The clutch coil is exposed to heat and vibration at the front of the engine, and its internal windings can short out over time, causing an excessive current draw that blows the fuse. This is the most common root cause for P0645 on this platform.
   How to confirm: After replacing the blown fuse #37, if it blows again, disconnect the electrical connector at the A/C compressor. Replace the fuse one more time. If the fuse no longer blows, the A/C compressor clutch coil is confirmed to be shorted.
   Typical fix: Replace the A/C compressor assembly or, if available, the A/C compressor clutch and coil. Most owners and shops replace the entire compressor assembly.
   Est. part cost: $150-$350
3. Faulty A/C Clutch Relay 🟡 Medium Probability → Shop Wiring Relay
   How to confirm: Locate the A/C clutch relay in the underhood fuse box. Swap it with an identical relay from a non-essential system (like the horn) and see if the A/C engages or if the fuse issue is resolved.
   Typical fix: Replace the A/C clutch relay.
   Est. part cost: $10-$30
4. Wiring Harness Damage ⚪ Low Probability
   How to confirm: Visually inspect the wiring between the fuse box, the A/C relay, and the A/C compressor for any signs of chafing, melting, or corrosion. If the fuse still blows with the A/C compressor disconnected, the short is in the wiring harness.
   Typical fix: Repair or replace the damaged section of the wiring harness.
   Est. part cost: $5-$100

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): This is very rare. The PCM's internal driver for the A/C relay can fail, but all other potential causes, especially the fuse and A/C clutch, should be ruled out first.

Diagnosis Steps

1. Attempt to start the vehicle. Note if it cranks but fails to start.
2. Scan for OBD-II codes to confirm P0645 is present.
3. Open the underhood fuse panel and locate the A/C BFC fuse at position #37 (a 10-amp fuse).
4. Visually inspect the fuse. If it is blown, replace it.
5. Attempt to start the engine with the new fuse. If it starts and runs, the immediate issue is resolved. Proceed to find the cause of the blown fuse.
6. Turn the A/C system on. If the fuse blows immediately, the problem is likely a shorted A/C compressor clutch.
7. To confirm the shorted clutch, disconnect the electrical connector at the A/C compressor.
8. Replace the fuse again and start the engine. If the fuse does not blow, the A/C compressor clutch is faulty and the compressor assembly needs to be replaced.
9. If the fuse was not blown initially, test the A/C clutch relay by swapping it with a known good, identical relay (the horn relay is often a suitable donor for testing).
10. If the relay and fuse are good, inspect the wiring harness for damage. If no damage is found, further diagnosis of the PCM control circuit may be needed.

Parts You'll Likely Need

• A/C Compressor (OEM #ACDelco 15-22239 (V6), ACDelco 15-22273 (4-cyl)) — The internal clutch coil is the most common part to short-circuit, which blows the fuse that also controls engine starting functions. The clutch is often replaced as part of the full compressor assembly.
  Trusted brands: ACDelco, Delphi, Four Seasons, Denso, UAC
  OEM price range: $250-$400
  Aftermarket price range: $150-$350
• Mini Fuse (10 Amp) — This fuse in position #37 is the first component to fail when the A/C clutch shorts out. It must be replaced, but it will blow again if the underlying short is not fixed.
  Trusted brands: Bussmann, Littlefuse
  OEM price range: $1-$5
  Aftermarket price range: $1-$5
• A/C Clutch Relay (OEM #Standard Motor Products 89133NGFQ) — The relay can fail internally, causing the circuit to malfunction. It's an inexpensive and easy part to swap for diagnosis.
  Trusted brands: ACDelco, Standard Motor Products
  OEM price range: $15-$40
  Aftermarket price range: $10-$30

Technical Service Bulletins (TSBs) & Recalls

• PIP5352: Addresses a crank-no-start condition with no fuel injector pulse, linking it to DTC P0645 and a blown fuse in the under hood fuse panel at position #37, caused by a faulty A/C compressor clutch coil.

Platform-Specific Known Issues

• A documented issue exists where a fault in the A/C clutch circuit will blow fuse #37, which also disables the fuel injector pulse, resulting in a crank-no-start condition. This is detailed in GM Technical Service Bulletin #PIP5352.
• Owner Experience: Crank No Start, No Codes Initially: An owner on a GM forum described a 2002 Alero with an intermittent crank-no-start condition. After replacing numerous parts (fuel pump, crank/cam sensors, ECM), the issue persisted. This highlights how the P0645-related fuse issue can be misdiagnosed as a more complex fuel or ignition system failure.

Mechanic-Grade Diagnostic Values

• A/C Compressor Clutch Coil Resistance — expected: 3.0 to 5.0 Ohms (Ω). Failure: A reading near 0 Ω indicates a short circuit. A very high or infinite reading indicates an open circuit.
• Voltage at A/C Compressor Clutch Connector (A/C commanded ON) — expected: Within 1.0 volt of battery/charging system voltage (approx. 12-14.5V). Failure: A voltage drop greater than 1.0 volt indicates excessive resistance in the power or ground side of the circuit.
• A/C Clutch Relay Coil Resistance — expected: ~50–200 Ohms (Ω). Failure: A reading outside the manufacturer's specification indicates a faulty relay coil.

Scan Tool Commands That Help

• GM Tech 2: A/C Clutch Relay Control — Use this bidirectional control to manually command the A/C clutch relay on and off. This helps isolate the fault by verifying if the PCM can control the relay and if the relay and clutch respond. It separates a control-side issue from a load-side (clutch/wiring) issue. The path is typically Powertrain > Special Functions > Engine Output Controls > A/C Relay.

Wiring & Ground Locations

• G103 — On the left side of the engine compartment, on the cowl, above the brake booster. On some engine variants, it may be on a transmission stud on the left side of the engine.. This ground point serves multiple critical components, including the Powertrain Control Module (PCM) and Body Control Module (BCM). A poor connection at G103 can cause erratic behavior in the control modules, potentially leading to false codes or incorrect operation of circuits like the A/C clutch relay control.
• Underhood Fuse Block — Located in the engine compartment on the driver's side, near the strut tower.. This block contains both the A/C BFC Fuse (#37) and the A/C Compressor Relay. All primary power and control wiring for this circuit runs through this component, making its connectors (like C1 and C2) and internal busses potential points of failure.

Real Owner Repair Stories

• CarComplaints.com user report (2003 Oldsmobile Alero) — Car acted as if it was out of gas and would not start. It would crank but not turn over.
  ❌ Tried (didn't work) Using the Passlock anti-theft reset procedure (key on for 10 min)., Checking fuel-related fuses and relays.
  ✅ What actually fixed it A garage discovered a leaky water pump was dripping onto the A/C compressor, causing it to short out. This blew the 10-amp A/C fuse (also labeled BFC for Body Function Control), which in turn cut power for fuel flow, causing the no-start condition. Replacing the fuse and addressing the short (and the water pump) fixed the issue.
• YouTube video by user 'It's Don' (2003 Oldsmobile Alero 3.4L) — Intermittent crank-no-start, particularly when the engine was hot. Would start with starter fluid, indicating a fuel delivery issue.
  ❌ Tried (didn't work) Replacing the fuel pressure regulator., Replacing the mass airflow meter.
  ✅ What actually fixed it The user identified the blown 10-amp A/C fuse in the underhood fuse box as the direct cause of the no-start. After replacing the fuse, the car started. He confirmed the root cause was the A/C compressor by disconnecting it, which prevented the fuse from blowing again.