Daikin U4 is a communication fault on the serial bus connecting indoor and outdoor units. The most common cause is a loose terminal screw at either control board. Check for loose push-in connectors or voltage drops on long wire runs.
U4 basically shuts your whole system down until the indoor and outdoor units can talk again. I've seen homeowners ignore this for days thinking it'll reset itself, and it won't. Could be a $0 fix if it's just a loose terminal screw, or it could mean a failed inverter board that's going to cost you real money. Don't sit on this one.
Multimeter (DC voltage and continuity modes), Phillips #2 screwdriver
What Does the U4 Code Mean?
OK so here's the deal with U4. Your indoor and outdoor units talk to each other over a 2-wire signal line, and something broke that conversation. Could be a $0 fix (loose screw) or a $300+ fix (bad control board). Honestly, I'd say 60% of U4 calls I go on are just a loose terminal connection. Don't panic yet. Start at the wire and work your way toward the expensive stuff.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Loose S1-S2 terminal screws at indoor or outdoor unit40%
Rodent damage to communication wire in wall chase24%
Communication wire exceeding maximum length causing voltage drop14%
Indoor PCB board failure12%
Outdoor unit control board failure10%
Symptoms You May Notice
U4 flashing on the indoor display and the whole system's dead, no heating, no cooling, nothing running at all.
Indoor fan might still spin and push air around but the compressor outside never kicks on, so you get airflow but zero temperature change.
Outdoor unit is completely silent and dark, no LED indicators, no fan spinning, just sitting there doing nothing.
Error comes and goes depending on wind or weather because the signal wire's flexing inside the wall chase and making intermittent contact.
You reset the breaker, it runs fine for 15-20 minutes, then U4 comes right back.
Can you reset a Daikin minisplit to clear the U4 code?
Flip the circuit breaker off and leave it off for at least 60 seconds. Daikin inverter boards take longer to fully discharge than most people think, 30 seconds isn't always enough. Restore power, give it 2-3 minutes to boot up completely before touching the remote. Press Cancel on the remote for 5 seconds to clear the fault memory. If U4 comes right back, there's still a real problem, it's not a random lockout that reset will fix.
Tools Required for Diagnosis
Multimeter (DC voltage and continuity modes)Phillips #2 screwdriverFlathead screwdriverWire strippersNeedle-nose pliersNon-contact voltage tester
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
ComponentComponent Under Test
Expected Range0–30 VDC
ConditionIf Open (OL) or infinite, replace component.
Replacement Parts
If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.
Part Name
OEM Number
Estimated Price
Daikin Outdoor Unit Control BoardModel-specific · $150–$350
Model-specific
$150 – $350
Frequently Asked Questions
Can I fix Daikin U4 myself?
You can definitely handle the wiring inspection yourself. Start by tightening the S1 and S2 screws at both ends and do a visual check on the entire wire run for damage. If all the connections are solid and U4 comes back after a reset, that's when it gets complicated. A failed outdoor inverter board involves high-voltage DC circuits and a board configuration step on installation. That part I'd leave to a certified Daikin tech. The wiring diagnostics though? Totally doable yourself.
How much does it cost to fix a U4 error?
Depends entirely on what's wrong. Loose screw costs you nothing but 20 minutes. Broken communication wire is maybe $20-50 for wire plus an hour of your time. A failed outdoor inverter PCB is where it hurts, those parts run $150 to $400 depending on the model, plus 1-2 hours of labor if you're paying a tech. Indoor PCBs are similar money. Before you buy anything, do the diagnostics. I've ordered parts based on assumptions before and felt pretty dumb when the fix turned out to be tightening a screw.
Why does U4 keep coming back after I reset it?
Something's intermittent. Usually it's a loose screw that's just barely making contact, or a wire with a partial break that only opens up when the wire flexes or heats up a little. Could also be the wire run is too long for the gauge you used and you're getting signal dropouts under certain conditions. Try physically wiggling the wire at both ends while the unit's running to see if you can trigger the fault. That'll tell you pretty quickly whether it's a physical connection problem or a board issue.
What wire spec does Daikin require for the S1/S2 communication run?
Daikin calls for 18 AWG 2-conductor shielded wire for the S1/S2 run. Maximum run length is typically 65 feet (20 meters) at that gauge. Some systems support longer runs but you'd need to verify with Daikin for your specific model. The wire is polarity-sensitive on most models, so S1 connects to S1 and S2 to S2 at both ends. Crossing them won't cause a spark or anything, but it'll definitely cause U4 and you'll spend an hour chasing a problem that's a 10-second fix once you figure it out.
Can running the system with a U4 error cause damage?
The system won't actually run in U4 lockout, it's a full shutdown, so you're not going to burn out the compressor or anything like that. It's a safety feature doing its job. That said, if you keep forcing resets over and over without fixing the root cause, you can stress the control boards over time, especially if there's an underlying electrical issue causing noise on the signal wire. I wouldn't ignore it for more than a week or two. Fix the communication issue and let the system run clean.