High deviation of flow sensor and safety flow sensor detected
The 73 error code on Immergas boilers indicates high deviation of flow sensor and safety flow sensor detected.
Immergas 73 means: high deviation of flow sensor and safety flow sensor detected. On this component the documented failure modes are the board detects an anomaly in the temperature reading of the delivery ntc sensor and the causes may be: faulty sensor, incorrect positioning, poor system circulation, clogging on the water side of the primary exchanger, delivery sensor faulty or incorrectly inserted or the flow sensor has failed — short circuit or open circuit. You can safely reset the boiler once — if 73 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does 73 Mean
The 73 code is shown by the boiler’s control board when it detects high deviation of flow sensor and safety flow sensor detected. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows 73 and the boiler stops until the fault is cleared |
Common Causes
The first cause below is the one the Immergas service literature publishes for this code (source: centriassistenza.org). The others are the failure modes documented for the same component on other boilers, confirmed by 6 different manufacturers.
| Cause |
|---|
| The board detects an anomaly in the temperature reading of the delivery NTC sensor and the causes may be: faulty sensor, incorrect positioning, poor system circulation, clogging on the water side of the primary exchanger |
| Delivery sensor faulty or incorrectly inserted |
| The flow sensor has failed — short circuit or open circuit |
| Wiring, connector or harness of the flow sensor damaged or loose |
Safety Warning
73 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
How to Fix It
If normal conditions are restored, the boiler starts again without the need to be reset
If normal conditions are restored, the appliance restarts without the need to be reset
Measure the resistance of the flow sensor with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range.
Unplug and re-seat the connector of the flow sensor, then check the harness for chafing, corroded pins and breaks in continuity.
Refer to the Immergas service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
Diagnostic Tools
TroubleAtlas Score
What Is the TroubleAtlas Score?
Every boiler model is independently evaluated and scored by TroubleAtlas across 6 dimensions: reliability, efficiency, parts availability, service network, value for money, and user satisfaction. The score helps you compare models and make informed decisions when choosing a new boiler.
See All Boiler Scores →Boiler Breakdown Cover
Boiler Breakdown Cover
Protect against unexpected repair costs. Cover plans include annual service, emergency callouts within 24 hours, and full parts replacement. Compare plans side by side and choose the level of protection that fits your home and budget.
Coverage Plans — Coming SoonConsidering a New Heating System
Considering a New Heating System?
If your boiler or heat pump is over 10 years old, replacing it with a modern condensing boiler or air-source heat pump could save you up to 30% on energy bills and significantly reduce your carbon footprint.
Compare Heating System Prices →
YouTube Channel
Watch the fixes. Follow the channel.
Step-by-step boiler repair guides, equipment reviews, and heating system comparisons from qualified engineers.
New fixes every weekFix in 5
Quick fixes for common boiler faults explained in under 5 minutes
Boiler Comparator
Side-by-side comparisons of popular boiler models
Boiler Reviews
Independent reviews with TroubleAtlas Score ratings
Find a Specialist
Don’t fix it alone. Find a trusted heating engineer near you.
TroubleAtlas connects you with vetted local heating professionals who can inspect, diagnose and repair your boiler safely — no guesswork, no unnecessary call-outs.
Local heating professionals are coming to your area
We’re onboarding Gas Safe registered and licensed HVAC engineers city by city. Check back soon, or search above to see current availability.
TroubleAtlas Pro
Get cited by AI. Get found by customers.
TroubleAtlas Pro puts your business in front of homeowners searching for exactly this fault, and structures your listing so AI assistants can cite you as a trusted source.
- ✓ Appear on error code pages viewed by thousands of homeowners
- ✓ AI assistants cite your listing as a verified local specialist
- ✓ Receive direct leads with customer name and contact details
- ✓ TroubleAtlas Certified badge for your marketing materials
Affected Models
Immergas models that can display fault code 73:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Immergas models | — | Possible |
Frequently Asked Questions
If normal conditions are restored, the boiler starts again without the need to be reset. According to the technical documentation, this is the first check on the list for 73.
73 is not classified as a high-severity fault, so the boiler can be reset once. If 73 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.