P0645 on Honda: How to Diagnose & Fix
A/C Clutch Relay Control Circuit
2 Honda models · 2 documented issues · $250-$900 repair
P0645 on Honda by model: symptoms, diagnosis and fix
The Omron A/C clutch relay (Honda part 39794-SDA-A03) is defective and gets hot, causing failure. Honda-specific part with reduced coil current known to be problematic. When relay fails, either battery dies overnight (relay stuck closed) or AC stops working (relay stuck open). Low refrigerant from pressure relief valve venting can lead to failed AC compressor.
Common Symptoms
- AC not blowing cold air
- AC works intermittently
- Battery drain overnight when parked
- AC compressor not engaging (no click when AC turned on)
- Squeaking noise from compressor clutch
- Low refrigerant / refrigerant leak
How to Fix
Replace defective Omron relay (39794-SDA-A03) with updated Mitsuba relay (39794-SDA-A05) at dealer ($20-$40 part + $50-$100 labor) or DIY (relay in engine bay fuse box). If AC still not working, check refrigerant level and recharge ($150-$200). If clutch physically failed, full compressor replacement required ($1,200-$1,900).
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Honda/Mitsuba 39794-SDA-A05 power relayOEMHonda 39794-SDA-A05
Honda Pilot applications; the 2016 Pilot engine-room catalog explicitly identifies 39794-SDA-A05 as the magnetic-clutch relay. Confirm VIN, fuse-box position
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Owner tips & cautions
- TipEasy DIY repair - relay located in engine bay fuse box, takes 5 minutes to replace
- TipCheck for battery drain overnight - if present, AC clutch relay likely stuck closed
- WarningLow refrigerant from failed relay can damage AC compressor - replace relay immediately when AC stops working
Sources for this issue (1)
8th generation Accords (especially V6) commonly lose A/C because the compressor clutch fails to engage. The magnetic clutch coil opens (loses continuity) or the clutch air gap widens with wear, so the clutch plate no longer pulls in when the A/C is requested - the compressor pulley spins but the compressor never turns. A failed high/low pressure switch or the 7.5A MG-clutch fuse can produce the same no-engage symptom. The result is warm air from the vents even with a full refrigerant charge. This is the compressor-clutch engagement failure, distinct from the refrigerant-loss condenser-leak issue.
Common Symptoms
- A/C blows warm despite full charge
- Compressor clutch does not click/engage when A/C is on
- Compressor pulley spins but center clutch plate stays still
- Intermittent cooling (clutch slips)
- Blown MG-clutch fuse
How to Fix
Confirm the clutch coil with an ohmmeter (good coil ~3-4 ohms; open = failed). Check the 7.5A MG-clutch fuse (#20) and the A/C pressure switches first. Remedy is replacing the clutch coil/clutch assembly, or re-shimming to correct the air gap; if the compressor itself is worn, the full compressor is replaced. Dealer clutch+stator jobs run ~600-900 USD; independent clutch coil replacement is cheaper.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine 2.4L A/C compressor clutch field coilOEMPartsGeek Genuine W0133-2153003
Confirm compressor manufacturer/model, connector and VIN before purchase.
- Honda 38924-R70-A01 A/C compressor clutch field coil/statorOEMHonda 38924-R70-A01
Replace an electrically failed field coil on the V6 branch Fitment/availability: 2008-2012 Accord 3.5L V6 applications cataloged for 38924-R70-A01 after confirming body/transmission, compressor and connector by VIN.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (1)
P0645 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0645 mean on a Honda?
P0645 stands for "A/C Clutch Relay Control Circuit." A general fault in the A/C compressor clutch relay control circuit driven by the control module. The commanded relay state does not match the measured circuit condition. On Honda it is documented across 2 models: Pilot, Accord.
What are the most common causes of P0645?
The most common causes of P0645 are: Faulty A/C clutch relay, Open or short in the relay control circuit, Corroded/damaged connectors, Blown fuse, Internal PCM driver fault. The specific cause varies by vehicle.
How much does it cost to fix P0645 on a Honda?
Published repair estimates on Honda range from $250 to $900 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0645 on a different make?
View P0645 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.