🔧 AC Cooling Only Sometimes? Complete Guide to Fix Thermostat & Sensor Problems
Problem Overview
Your AC works perfectly for a while, then suddenly stops cooling or blows warm air, only to start working again later. This frustrating intermittent problem is often caused by faulty thermostats misreading room temperature, defective temperature sensors, control board glitches, loose electrical connections, or compressor short-cycling issues. This comprehensive guide helps you diagnose the exact cause and fix the problem.
Safety First
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Electrical hazard: Always turn off AC at the circuit breaker before opening any panels or inspecting internal components. Thermostats and sensors work with live electrical connections.
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Never bypass sensors: Disconnecting or bypassing temperature sensors can cause compressor damage, refrigerant leaks, or fire hazards. Sensors are safety devices—always repair or replace properly.
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Control board caution: PCB (Printed Circuit Board) contains sensitive electronics. Avoid touching components while powered. Water, static electricity, or short circuits can damage expensive boards.
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Warranty consideration: Opening sealed units or tampering with sensors/thermostats may void manufacturer warranty. Check warranty status before attempting repairs—call authorized service if under warranty.
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Refrigerant system: If intermittent cooling involves refrigerant issues (pressure fluctuations, leaks), only certified technicians should handle. Never attempt refrigerant work yourself.
🔍 Quick Diagnostic Checks ⏱️ 5-10 minutes
1 Note the Cooling Pattern
Track when AC stops cooling: after specific duration (20 min, 1 hour), at certain times (afternoon peak), or completely random. Pattern reveals sensor vs mechanical issue.
2 Check Display Temperature
Compare displayed room temperature vs actual (use separate thermometer). If display shows wrong temp (26°C when actually 32°C), sensor is faulty.
3 Listen for Compressor Cycling
Go to outdoor unit when cooling stops. If compressor turned off (no hum/vibration) but shouldn't have, indicates thermostat or sensor told it to stop prematurely.
4 Test Remote Control
Replace batteries in remote. Try setting different temperatures. If AC responds inconsistently or display flickers, remote or receiver circuit may be failing.
5 Check for Error Codes
Modern ACs display error codes on LED panel. Common codes: E1/E2 (sensor), E4 (outdoor unit), E6 (communication). Note code and check manual for meaning.
6 Inspect Electrical Connections
Power OFF first. Check visible wire connections at indoor unit. Loose wiring causes intermittent operation—wires disconnect/reconnect with vibration causing random cooling stops.
💡 Common Causes → Solutions
Problem
Faulty temperature sensor reading incorrectly
Solution
Sensor replacement by technician (₹800-₹2,500). Sensor tells AC when room reached set temperature. Faulty sensor gives wrong readings causing premature compressor shutdown.
Problem
Thermostat malfunction or incorrect calibration
Solution
Reset AC to factory settings first. If issue persists, thermostat recalibration or replacement needed (₹1,500-₹4,000). Modern inverter ACs have digital thermostats in PCB—requires professional diagnosis.
Problem
Loose electrical connections or oxidized contacts
Solution
Technician tightens all electrical terminals, cleans oxidized contacts, checks wire insulation. Vibration loosens connections over time—causes intermittent power loss to compressor/sensors. Quick fix, usually ₹500-₹1,000.
Problem
Compressor overheating and thermal protection tripping
Solution
Clean outdoor condenser coils to improve heat dissipation. Ensure proper clearance around outdoor unit. Check for low refrigerant (causes overheating). If compressor still overheats, may need capacitor replacement or compressor servicing.
Problem
PCB (control board) malfunction or software glitch
Solution
Power cycle AC completely (off at breaker for 10 minutes). This resets PCB. If problem persists, board repair (₹2,000-₹4,000) or replacement (₹3,500-₹8,000) needed. Check for voltage fluctuation damage.
Problem
Voltage fluctuations causing compressor to cut out
Solution
Install voltage stabilizer (₹2,000-₹5,000) with appropriate capacity (0.5kVA per ton). Check if voltage drops during peak hours. Many inverter ACs have built-in low voltage protection that shuts compressor down.
Problem
Failing compressor capacitor causing intermittent starts
Solution
Weak capacitor can't consistently start compressor. Symptoms: compressor attempts to start (humming), then stops. Capacitor replacement (₹300-₹800) solves issue. Simple, inexpensive fix if diagnosed correctly.
Problem
Refrigerant pressure fluctuations (partial leak or restriction)
Solution
Slow leak causes pressure to drop over hours, compressor stops. Pressure rebuilds slightly, compressor restarts briefly. Requires leak detection, repair, vacuum, and proper refill (₹2,500-₹6,000). Professional service only.
🛠️ Step-by-Step DIY Troubleshooting
Document the Intermittent Pattern
• Track exactly when cooling stops: note time, duration AC ran, room temperature
• Record how long cooling is off before it resumes (5 min, 30 min, hours)
• Note if pattern is consistent (every 20 minutes) or random
• Check if problem worse at specific times (afternoon peak heat, evening)
• Use phone to record observations—helps technician diagnose if DIY fails
• Regular pattern (every X minutes) = sensor/thermostat; Random = electrical/mechanical
Verify Temperature Sensor Accuracy
• Get accurate digital thermometer to measure actual room temperature
• Place thermometer near AC indoor unit (where sensor is located)
• Compare thermometer reading vs AC display temperature
• Significant difference (5°C+) confirms faulty sensor giving wrong readings
• AC thinks room is cooler than reality, shuts compressor off prematurely
• Test multiple times throughout day for consistency
Perform Complete Power Reset
• Turn off AC using remote control
• Go to electrical panel and turn off AC circuit breaker
• Wait full 10 minutes (allows capacitors to fully discharge, PCB to reset)
• Turn breaker back on, wait 2 minutes before starting AC
• Start AC and monitor for next 2-3 hours to see if intermittent issue resolved
• Power reset clears software glitches, resets sensor calibration, clears error states
Test Remote Control Thoroughly
• Replace remote batteries with fresh ones (weak batteries cause erratic signals)
• Clean remote sensor (LED at front) with cotton swab and alcohol
• Point remote directly at AC receiver, ensure no obstructions
• Test by setting different temperatures, modes—AC should respond immediately
• If display shows incorrect commands or intermittent response, remote/receiver faulty
• Try manually controlling AC from indoor unit panel if available
Inspect Visible Electrical Connections
POWER OFF AT BREAKER FIRST—CRITICAL SAFETY STEP
• Remove indoor unit cover/panel (usually slides up or has side clips)
• Visually inspect wire connections to terminal block
• Look for loose wires, burnt marks, discoloration, or corrosion
• Check wire insulation for cracks or exposed copper
• Gently wiggle wire connectors—should be firm, not loose
• If confident, tighten visible terminal screws (don't overtighten)
Check for Compressor Overheating Protection
• When cooling stops, immediately go to outdoor unit
• Carefully feel outdoor unit metal casing—if extremely hot, compressor overheating
• Compressors have thermal cutout that stops operation when too hot
• After cooling down (30-60 min), compressor restarts—explains intermittent cooling
• Causes: dirty condenser coils, low refrigerant, failing capacitor, blocked airflow
• Clean outdoor coils, ensure 2 feet clearance, check for debris blocking fins
Decode Error Codes (If Displayed)
Common error codes related to intermittent cooling:
• E1/E2: Room temperature sensor fault—sensor replacement needed
• E4/E5: Outdoor unit sensor/communication error
• E6/E7: Communication error between indoor/outdoor units
• F0/F1: High pressure protection (refrigerant issue)
• P0/P4: Inverter module or IPM failure (control board)
• Note exact code and check AC manual or manufacturer website for specific meaning
Test Voltage Stability
• Use digital multimeter to check AC outlet voltage (220-240V for India)
• Measure voltage when AC is OFF, then when running (voltage shouldn't drop much)
• If voltage drops below 190V when AC runs, insufficient power supply
• Check if cooling stops correlate with high power usage times (evening peak)
• Many ACs have low voltage cutoff (185-190V)—compressor won't run below this
• Voltage drops below cutoff = compressor stops; voltage recovers = compressor restarts
Monitor Compressor Start/Stop Behavior
• Set AC to coldest setting (16-18°C) and maximum fan speed
• Stay near outdoor unit and listen carefully to compressor operation
• Normal: Compressor runs steadily for 15-20 minutes minimum before cycling off
• Abnormal: Compressor runs 2-5 minutes then stops (short cycling)
• Very abnormal: Compressor struggles to start (humming/clicking), then gives up
• Short cycling indicates thermostat/sensor issue; start struggles = capacitor/electrical issue
Factory Reset and Recalibration
• Locate factory reset procedure in AC manual (varies by brand/model)
• Common method: Press and hold specific button combination on remote (usually Mode + Temp Down for 5 sec)
• Alternative: Use reset button on indoor unit PCB (small pinhole button)
• Factory reset clears all settings, sensor calibration, error history
• After reset, reconfigure AC: set temperature, mode, test operation for 2-3 hours
• If intermittent issue resolved, was software/calibration problem; if continues, hardware fault
📞 When to Call a Professional
🔬 Sensor & Thermostat Issues
- • Display shows incorrect temperature vs actual (5°C+ difference)
- • Error codes E1, E2, or sensor-related codes displayed
- • AC cooling stops at regular intervals (same duration every time)
- • Factory reset and calibration don't solve the problem
- • Temperature sensor physically damaged or disconnected
⚡ Electrical & Component Failures
- • Burnt smell or visible burn marks on wiring/components
- • Compressor humming but not starting (weak/failed capacitor)
- • Tripping circuit breaker intermittently
- • Loose connections you're not comfortable repairing
- • Control board/PCB displaying error codes
💰 Expected Repair Costs (2025 India)
Thermostat Repair/Replace: ₹1,500-₹4,000
Remote Control: ₹500-₹1,500
PCB Replacement: ₹3,500-₹8,000
Compressor Capacitor: ₹300-₹800
Voltage Stabilizer: ₹2,000-₹5,000
Diagnostic Service: ₹500-₹1,000
🔍 Questions to Ask Technician
- • What is exact diagnosis? Ask for specific component failure (sensor, PCB, capacitor, etc.)
- • How did you test the sensor/thermostat? (Should use multimeter, temperature calibration test)
- • Can you show me the faulty component? (Burnt capacitor, damaged sensor, etc.)
- • Is this repair or replacement? If replacement, why can't component be repaired?
- • What's warranty on new parts? (Genuine parts should have 6-12 month warranty)
- • Will this fix the intermittent issue permanently or just temporarily?
- • Are there other failing components that need attention soon?
📊 Diagnostic Clues: Pattern Recognition Table
| Symptom Pattern | Likely Cause | DIY Possible? | Typical Cost |
|---|---|---|---|
| Stops cooling every 15-20 min consistently | Faulty temperature sensor | No | ₹800-₹2,500 |
| Random stops, no pattern | Loose electrical connections | Maybe | ₹500-₹1,500 |
| Stops after 30-60 min, resumes after cooling | Compressor overheating | Partial | ₹300-₹3,000 |
| Stops during peak evening hours only | Voltage fluctuation/drop | Yes | ₹2,000-₹5,000 |
| Compressor hums but doesn't start | Failed/weak capacitor | No | ₹300-₹800 |
| Error codes displayed, then clears | PCB/control board glitch | Reset only | ₹2,000-₹8,000 |
| Display shows wrong temperature | Sensor calibration/failure | No | ₹800-₹2,500 |
| Stops after few hours, runs fine when cool outside | Undersized AC/heat load issue | Improve room | Varies |
🛡️ Preventive Care & Maintenance
Monthly Checks
Clean filters, test temperature accuracy, listen for unusual sounds, check for error codes, ensure outdoor unit clear of debris
Annual Professional Service
Sensor calibration check, thermostat accuracy test, electrical connection tightening, capacitor testing, PCB inspection, voltage verification
Early Warning Signs
Inconsistent cooling performance, temperature display fluctuations, longer cooling cycles, unusual compressor behavior, sporadic error codes
💡 Prevention Tips for Sensor/Thermostat Longevity:
- • Voltage stabilizer: Protects sensitive electronics (PCB, sensors) from voltage spikes—adds 3-5 years to component life
- • Regular cleaning: Dust accumulation on sensors affects accuracy—clean indoor unit interior annually
- • Avoid moisture: Keep indoor unit dry—water damage is common cause of sensor/PCB failure
- • Proper installation: Ensure sensor not near heat sources (sunlight, lamps, kitchen)—causes false readings
- • Software updates: Some brands offer firmware updates for inverter ACs—improves sensor logic and reliability
- • Quality remote batteries: Use quality alkaline batteries—leaking batteries can damage remote/receiver circuit
- • Annual calibration: Professional sensor calibration ensures accurate temperature control—prevents premature wear
- • Power surge protection: Lightning, grid surges damage PCBs—use proper grounding and surge protectors
📋 Sensor Health Monitoring Checklist
✓ Compare AC display temp vs room thermometer
✓ Check for consistent compressor cycling
✓ Verify remote control responsiveness
✓ Note any error code displays
✓ Test cooling consistency over 2 hour period
✓ Verify set temperature achieves actual cooling
✓ Check all modes (cool, dry, fan) work properly
✓ Ensure timer and sleep functions operate correctly
❓ Frequently Asked Questions
Q1: AC cools for 20 minutes then stops, resumes after 10 minutes. What's causing this cycle?
Classic symptom of faulty temperature sensor. Sensor incorrectly tells AC that room has reached set temperature after 20 minutes, compressor shuts off. Room heats up, sensor finally detects this, compressor restarts. This regular pattern (not random) indicates sensor giving false readings. Requires sensor replacement (₹800-₹2,500). Until fixed, manually restart AC when cooling stops.
Q2: Display shows room is 26°C but thermometer reads 32°C. Can I recalibrate sensor myself?
6°C difference confirms sensor malfunction. DIY recalibration not possible on most modern ACs—sensor is factory calibrated and sealed. Some older ACs have adjustment screw, but incorrect adjustment causes worse problems. Best solution: (1) Factory reset AC (may recalibrate), (2) If problem persists, sensor replacement by technician. Operating with faulty sensor wastes electricity and reduces comfort—get it fixed promptly.
Q3: AC works fine all day but stops cooling every evening between 7-9 PM. Why?
Time-specific issue indicates voltage problem, not sensor. During peak evening hours (7-9 PM), neighborhood power demand spikes, voltage drops. Many ACs have low voltage cutoff (185-190V)—if voltage drops below this, compressor won't run. When fewer appliances run later, voltage recovers, AC resumes. Solution: Install voltage stabilizer with adequate capacity (0.5kVA per ton AC). Cost: ₹2,000-₹5,000 but protects AC and ensures consistent operation.
Q4: Compressor runs for 5 minutes, stops for 2 minutes, repeats constantly. Is this normal?
Absolutely NOT normal—this is "short cycling" and very damaging to compressor. Causes: (1) Thermostat stuck in on/off loop, (2) Severe overheating causing thermal protection to trip repeatedly, (3) Low refrigerant causing pressure cutout cycling, (4) Electrical issue causing intermittent power loss. Turn AC off immediately—continued short cycling will burn out compressor (₹15,000+ replacement). Call technician urgently for proper diagnosis.
Q5: Factory reset fixed intermittent cooling for a week, now problem is back. What next?
Temporary fix from reset indicates hardware failure developing. Reset cleared software glitch or recalibrated sensor temporarily, but underlying component is failing. Common scenarios: (1) Sensor degrading—needs replacement, (2) PCB solder joints cracking—needs board repair, (3) Capacitor weakening—needs replacement. Don't keep resetting hoping for permanent fix. Call technician for proper component-level diagnosis and replacement. Delaying repair makes problem worse and costlier.
Q6: Can I bypass faulty sensor temporarily to keep AC running?
NEVER bypass temperature sensor—extremely dangerous and illegal in many places. Sensor prevents: (1) Compressor overheating, (2) Evaporator coil freezing, (3) Excessive cycling damage. Without sensor: compressor runs continuously → overheats → burns out (₹15,000+ loss). Or: evaporator freezes → copper pipe damage → refrigerant leak (₹5,000+ repair). Bypassing also creates fire hazard and voids ALL warranties. Proper fix is sensor replacement (₹800-₹2,500)—fraction of potential damage cost.
Q7: Remote control works but AC responds intermittently. Remote or AC problem?
Test to isolate: (1) Replace remote batteries with fresh ones, (2) Clean remote IR LED with alcohol, (3) Point remote directly at receiver without obstacles, (4) Use phone camera to verify IR signal transmitting. If remote tests good but AC responds sporadically, problem is in AC's IR receiver circuit or PCB. Receiver failure symptoms: commands work sometimes, especially when close; fail from distance or certain angles. Receiver replacement (₹800-₹2,000) or PCB repair (₹2,000-₹4,000) needed.
Q8: Brand new AC (3 months old) has intermittent cooling. Should I repair or demand replacement?
New AC should work flawlessly—intermittent issues indicate manufacturing defect or installation problem. Your rights: (1) Call authorized service immediately under warranty, (2) If same problem after 2-3 service visits, demand unit replacement (consumer protection law), (3) Don't accept "normal" or "will improve" excuses. Common new AC issues: improper installation, sensor calibration error, PCB defect. Insist on permanent fix or replacement. Document all service visits for warranty claims. New unit replacement is your right if defect can't be fixed.
Q9: AC cools perfectly in morning, fails in afternoon peak heat. Is AC undersized?
Maybe undersized OR sensor/component overheating. Diagnosis: (1) Measure temperature difference (room vs AC air) during both times. If delta-T similar (8-12°C) but room stays hot in afternoon, AC is undersized. If delta-T drops to 3-5°C in afternoon, components overheating. (2) Check outdoor unit during afternoon—if extremely hot, condenser/compressor overheating. (3) Feel refrigerant pipes—if hot instead of cool, refrigerant issue. Solution: Clean coils, improve ventilation, check refrigerant. If all OK but room won't cool, AC genuinely undersized—need higher tonnage unit.
Q10: Technician quoted ₹6,000 for "PCB replacement" for intermittent cooling. Is this justified?
PCB costs vary widely (₹3,500-₹8,000) depending on brand/model. Before accepting: (1) Ask for specific diagnosis—which component on PCB failed? (2) Request to see the error or failed component, (3) Ask if PCB repair possible instead of full replacement (₹2,000-₹4,000), (4) Get second opinion if quote seems high, (5) Verify it's genuine PCB, not local copy (voids warranty). Many issues misdiagnosed as "PCB problem"—actual fault could be sensor (₹2,500), capacitor (₹800), or connections (₹1,000). Insist on proper diagnosis before expensive PCB replacement.
⚠️ Disclaimer: This troubleshooting guide is for informational and diagnostic purposes only. Sensor replacement, thermostat repairs, control board work, and electrical troubleshooting require qualified technicians with proper testing equipment. Never bypass safety sensors or attempt electrical repairs without proper training—risk of fire, electric shock, equipment damage, or personal injury. If AC is under warranty, contact authorized service centers exclusively to preserve warranty coverage. When uncertain about any diagnostic step, or if problem persists after basic troubleshooting, always call a certified HVAC professional.
Important Safety Reminder
If you're not comfortable with any step, or if the problem persists after trying these solutions, please contact a qualified technician. Safety should always be your first priority when dealing with electrical appliances.
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