Hayward H-Series SF Error Code: Temperature Sensor Failure Diagnosis & Repair

Understanding the SF Code

The SF (Sensor Failure) code appears when the control board cannot read a valid signal from the water temperature sensor. The Universal H-Series uses a 10K ohm NTC (Negative Temperature Coefficient) thermistor to monitor water temperature. When this sensor reads outside expected parameters, the heater shuts down for safety.

The control board expects to see specific resistance values that correspond to water temperatures between 32°F and 150°F. Readings outside this range, or sudden jumps in readings, trigger the SF code.

How Temperature Sensors Work

The temperature sensor system includes:

• Thermistor sensor: Changes resistance based on temperature
• Sensor well: Dry pocket in water path where sensor mounts
• Two-wire cable: Carries resistance signal to control board
• Control board input: Converts resistance to temperature reading

As water temperature increases, the thermistor resistance decreases predictably. The control board applies a small voltage across the sensor and measures the resulting current to calculate resistance, then converts this to a temperature value.

Common Causes of SF Codes

Wiring and Connection Issues (60% of cases)

• Corroded connections: Moisture causing high resistance
• Loose wire terminals: Poor connection at sensor or board
• Damaged cable: Cut, pinched, or chewed wires
• Water intrusion: Moisture in connectors causing shorts

Sensor Problems (30% of cases)

• Failed thermistor: Internal open or short circuit
• Contaminated sensor: Corrosion on sensor body
• Poor thermal contact: Sensor not seated properly in well
• Wrong sensor type: Incorrect resistance specification

Control Board Issues (10% of cases)

• Damaged input circuit: Board cannot read sensor properly
• Corroded board connector: Poor connection at board
• Failed board components: Resistors or capacitors in sensor circuit

Temperature/Resistance Reference Chart

Temperature (°F)	Resistance (Ohms)	Temperature (°C)
32°F	32,650 Ω	0°C
50°F	19,900 Ω	10°C
68°F	12,500 Ω	20°C
77°F	10,000 Ω	25°C
86°F	8,057 Ω	30°C
95°F	6,531 Ω	35°C
104°F	5,327 Ω	40°C
122°F	3,602 Ω	50°C
140°F	2,488 Ω	60°C

Step-by-Step Troubleshooting

Initial Checks

1. Note Error Behavior

• Does SF appear immediately on power-up?
• Does it occur after heater runs for a while?
• Is the error intermittent or constant?
• Any recent work done on the heater?

2. Visual Inspection

• Check for obvious wire damage
• Look for moisture or corrosion at connections
• Verify sensor is properly plugged into board
• Check for rodent damage to wiring

Basic Testing (Homeowner Level)

3. Connection Check

• Turn off power at breaker
• Locate sensor connection at control board
• Unplug and replug connector firmly
• Look for bent or corroded pins
• Power on and test

4. Clean Connections

• If corrosion visible, clean with electrical contact cleaner
• Use fine sandpaper on corroded terminals
• Apply dielectric grease to prevent future corrosion

Advanced Testing (Technician Level)

5. Sensor Resistance Test

1. Turn off power and disconnect sensor from board
2. Set multimeter to resistance (20K ohm range)
3. Measure across sensor wires
4. Compare reading to temperature chart
5. Example: At 77°F water temp, expect ~10,000 ohms
6. Open circuit (OL) = failed sensor
7. Very low resistance (<100 ohms) = shorted sensor

6. Dynamic Sensor Test

1. Leave meter connected to sensor
2. Warm sensor with hand or warm water
3. Resistance should decrease smoothly
4. Erratic readings indicate bad sensor
5. No change indicates failed sensor

7. Wire Continuity Test

1. Disconnect sensor at both ends
2. Test continuity of each wire separately
3. Should show near 0 ohms
4. Test each wire to ground - should show open
5. Any shorts to ground will cause SF code

8. Substitute Sensor Test

1. Disconnect sensor wires from board
2. Install 10K ohm resistor across terminals
3. Power on heater
4. Display should show ~77°F
5. If SF code clears, sensor or wiring is bad
6. If SF remains, control board is suspect

Detailed Component Testing

Testing at the Sensor

1. Remove sensor from well (heater must be off)
2. Submerge in container of water with thermometer
3. Measure resistance at various temperatures
4. Create your own reference chart
5. Readings should match chart within ±5%

Testing at the Control Board

1. With sensor connected and system off
2. Back-probe connector at board
3. Measure resistance through entire circuit
4. Should match sensor-only reading
5. Higher resistance indicates wiring problem

Thermal Well Inspection

• Remove sensor from well
• Check for scale or corrosion in well
• Clean with wire brush if needed
• Apply thermal compound when reinstalling
• Ensure sensor bottoms out in well

Replacement Procedures

Sensor Replacement

1. Turn off power and allow system to cool
2. Drain heater below sensor level (if wet well)
3. Disconnect sensor wire from control board
4. Remove sensor from well (may require wrench)
5. Apply pipe tape or sealant to new sensor (if threaded)
6. Install new sensor, ensuring full insertion
7. Route wire away from hot surfaces
8. Connect to control board
9. Secure wire to prevent damage

Wire Harness Repair

1. Locate damaged section of wire
2. Cut out damaged portion
3. Strip wire ends 1/4 inch
4. Use waterproof butt splices or solder
5. Cover with heat shrink tubing
6. Secure repair away from moving parts

Common Replacement Parts

• Temperature Sensor (10K NTC): HADXSEN1930
• Sensor Wire Harness: HADXWIR1930
• Sensor Well Assembly: HADXWEL1930
• Control Board: Model-specific part number
• Thermal Compound: Generic heat sink paste

Prevention Strategies

• Apply dielectric grease to connections annually
• Secure wiring away from vibration and heat
• Install in dry, protected location when possible
• Use wire loom to protect against rodent damage
• Check connections during routine maintenance
• Keep control compartment dry and sealed
• Replace sensor preventively every 5-7 years in harsh environments

Troubleshooting Specific Scenarios

Intermittent SF Codes

Usually indicates:

• Loose connection that opens with vibration
• Corroded connection with varying resistance
• Wire damage that opens/closes with movement
• Moisture causing temporary shorts

SF Code After Rain

Points to:

• Water intrusion in control compartment
• Moisture in sensor connector
• Failed door seal allowing water entry

SF Code in Cold Weather Only

May indicate:

• Wire becoming brittle and cracking
• Connection contracting and losing contact
• Sensor reading below expected range

When to Replace Control Board

Consider control board replacement when:

• Sensor tests good but SF persists
• Resistor substitution doesn't clear code
• Multiple sensor replacements don't fix issue
• Visible damage to board sensor circuit
• Other erratic behavior accompanies SF code

← Back to Universal H-Series Troubleshooting Guide