Oil Furnace Starts Then Stops: Troubleshooting Causes and Reliable Fixes

The phenomenon where an oil furnace starts then stops can be frustrating and may signal issues ranging from simple maintenance needs to safety-related failures. This article outlines common causes, step-by-step diagnostics, and practical fixes to get a furnace running reliably while highlighting when to call a professional.

Symptom	Likely Cause	Quick Fix
Runs A Few Seconds Then Shuts Off	Faulty Flame Sensor, Air In Fuel Line, Or Ignition Problem	Clean Sensor, Bleed Line, Or Restart Ignitor
Starts Frequently (Short Cycling)	Thermostat Issues, Overheating, Or Clogged Filter	Check Thermostat, Replace Filter, Inspect Venting
Runs Then Locks Out	Pressure Or Flame Failure, Safety Switch	Check Oil Supply, Reset Lockout, Call Technician

How Oil Furnaces Operate: A Quick Overview

An oil furnace uses a burner assembly to atomize heating oil and ignite it to produce heat that circulates through the home via a blower and ductwork. Key components include the oil tank and supply line, oil pump, burner motor, nozzle, ignition transformer, flame sensor or cad cell, combustion chamber, heat exchanger, blower motor, thermostat, and various safety switches.

Understanding these components helps diagnose why an oil furnace starts then stops by showing which parts interact during the short run cycle and where failures typically occur.

Common Causes When An Oil Furnace Starts Then Stops

1. Flame Sensor (Cad Cell) Problems

The flame sensor, often a cadmium sulfide (cad cell) photocell, confirms the presence of flame. If it’s dirty, misaligned, or failing it may not detect flame and will shut the burner down quickly.

Signs: Furnace lights for seconds, then goes into lockout or repeats short cycles.

2. Ignition Or Transformer Failures

The ignition transformer provides the spark or high voltage to start combustion. A weak transformer or poor spark can produce intermittent ignition that fails after initial startup.

Signs: Clicking or sparking sounds, intermittent ignition, repeated starts and stops.

3. Fuel Delivery Issues (Air In Line, Clogged Filter, Empty Tank)

Oil delivery problems include a closed or clogged fuel line, dirty or clogged filter, vacuum in the line, or simply low or no oil in the tank. Air in the line causes sputtering ignition that stops when air reaches the burner.

Signs: Sputtering, smell of oil, burner primes repeatedly, or no steady flame.

4. Nozzle Or Pump Problems

The nozzle atomizes oil into a fine spray. A worn or clogged nozzle, or a failing oil pump, can reduce pressure and spray quality, causing the flame to be unstable and shutdowns to occur.

Signs: Uneven flame, smoke, sooting, or poor heat output followed by shutdown.

5. Overheating And Limit Switch Trip

If the furnace overheats due to blocked airflow (dirty air filter, closed vents) or blower failure, a high-limit switch will stop the burner to protect the system.

Signs: Furnace runs briefly, blower not running or airflow restricted, burner stops on limit trip.

6. Thermostat Or Control Board Issues

A malfunctioning thermostat or a failing control board can send intermittent or incorrect signals, causing the burner to start and stop unexpectedly.

Signs: Erratic on/off behavior, incorrect temperature readings, or control board fault lights.

7. Venting And Draft Problems

Poor chimney draft or blocked flue gases can cause combustion instability or safety switch activation, stopping the burner shortly after startup to prevent dangerous conditions.

Signs: Backdrafting smells, soot around the furnace or flue, or burner shutdown after ignition.

8. Safety Switches And Pressure Problems

Pressure switches, tank safety valves, and other interlocks detect unsafe conditions and will shut the system down when they sense an issue like excessive pressure or flame failure.

Signs: Lockout codes, tripped switches, or pressure-related error indicators.

Step-By-Step Troubleshooting Guide

Safety First

Before any inspection or repair, turn off power and follow manufacturer instructions. Working with fuel-burning equipment involves fire and carbon monoxide risks; call a licensed technician for any uncertain or complex repairs.

Basic Checks To Perform Before Calling A Pro

• Confirm the oil tank has fuel and supply valves are open.
• Check and replace the air filter if dirty to eliminate airflow restriction.
• Inspect the thermostat settings and batteries; set to heat with a temperature above room temp.
• Reset the burner lockout if the unit shows a lockout code; count the resets per manufacturer guidance.

Bleeding Air From The Oil Line

If suspected air in the line after an oil delivery, priming the pump and bleeding the line can help. This often requires a funnel or vacuum bleeder and following the furnace manual. If uncomfortable, contact an oil technician.

Cleaning Or Replacing The Flame Sensor

Turn off power, locate the cad cell, and clean it with a lint-free cloth; do not sand or scratch the sensor. If cleaning doesn’t help, replace the sensor. Proper alignment and wiring should be verified during reinstallation.

Inspecting The Nozzle, Pump, And Transformer

Inspect the nozzle for wear or clogging and replace it during annual service. Check pump pressure with a gauge; low pressure suggests pump or nozzle issues. Verify the ignition transformer is providing a strong, consistent spark.

Check For Overheating And Blower Operation

Ensure the blower motor runs when the furnace cycles and that vents are open. Replace filters regularly to prevent overheating limit trips. If the blower fails, the furnace will shut down shortly after starting to avoid overheating.

Test Thermostat And Control Board

Verify thermostat wiring and operation by calling for heat while observing furnace responses. Check control board status lights for error codes and consult the manual. Sometimes a simple thermostat replacement or control board reset resolves intermittent starts and stops.

Evaluate Venting And Draft

Inspect the chimney and flue for blockages, nests, or soot buildup. Ensure the draft inducer motor and flue connections are intact. Poor draft may require chimney cleaning or repair by a pro.

When Professional Service Is Recommended

Call a licensed heating technician if the furnace repeatedly locks out, if there are fuel leaks, persistent smoke or soot, strong fuel smells, or if troubleshooting steps don’t resolve the issue. Professionals can perform combustion analysis, adjust burner settings, replace components, and ensure safe operation.

Immediate professional service is required if there is any sign of carbon monoxide, strong fuel odor, or electrical burning smell.

Maintenance Tips To Prevent Short Cycling And Shutdowns

• Schedule annual professional tune-ups that include nozzle and electrode replacement, combustion testing, and pump pressure checks.
• Replace air filters every 1–3 months depending on use and home conditions.
• Keep the oil tank level monitored and use a quality filter at the tank and burner.
• Maintain clear access to vents and ensure proper chimney maintenance to avoid draft issues.
• Install carbon monoxide detectors and test them regularly for safety.

Costs And Parts To Expect

Typical service calls range widely; a basic diagnostic and minor repair can cost a few hundred dollars. Replacing items like a cad cell, nozzle, or filters is relatively inexpensive, while oil pump, transformer, or control board replacements can be significantly higher.

For budgeting purposes, homeowners should expect common part costs such as: nozzle $20–$50, cad cell $30–$120, oil pump $200–$600, control board $150–$400, plus labor.

FAQs: Quick Answers For Common Concerns

Why Does The Furnace Start Then Stop After A Short Time?

Short runs are usually due to flame detection failure, fuel delivery issues, or safety switches tripping from overheating or poor combustion.

Can Homeowners Fix This Themselves?

Homeowners can perform basic checks like replacing filters, verifying oil supply, and resetting lockouts. Tasks involving fuel lines, combustion adjustments, or electrical components should be left to licensed technicians.

How Often Should An Oil Furnace Be Serviced?

Annual service before the heating season is recommended. More frequent checks may be needed if problems recur or the system is older.

Key Takeaways For Homeowners

When an oil furnace starts then stops, the most common culprits are flame sensor faults, fuel delivery issues, ignition problems, and overheating from restricted airflow. Regular maintenance and prompt attention to unusual signs reduce the chance of system lockouts and costly repairs.

Document symptoms, perform safe preliminary checks, and call a qualified technician for combustion testing or complex repairs to ensure safe, reliable heating.