Heart rate monitor not right when working out

What sort of workout? How tightly do you wear the watch? How cold/hot is the environment?

You need to strap the watch down tightly so that it's not moving around on your wrist.

My experience is that it measures accurately for probably 98% of my workouts, and these include steady state running on all kinds of weather and rock climbing. People have reported issues with cycling and bar work at the gym, but this is an issue with all optical sensors.

It shouldn't matter what sport you select the watch should read your heart rate! My problem is if I don't pick a sport my Fenix 7 won't go above about bpm, but as soon as I remember and select a sport it reads more accurately. While my first fenix 7 went back when I reported this as an issue and Garmin said " that's not right we'll replace it under warranty" and I used a Pixel watch 2 for 10 days which measured heart rate all the time regardless of what was selected on the watch, also my Fenix 3 didn't need a sport to be selected. 

I'm white non hairy the strap is snug (works fine with a sport selected) size 10 feet and a cyclist and gym bunny (not bar work)

Boggy997 said:

It shouldn't matter what sport you select the watch should read your heart rate!

Wrist Flexing Types of Activities Can Affect Heart Rate Tracking

Activities that flex the wrist may create an effect similar to wearing the watch too tightly and restrict blood flow. Examples include, but are not limited to:

• Burpees

• Gripping Handlebars on a Bicycle

• Push-Ups

• Racket Sports

• Rowing

• Using Hiking Poles

• Weight Lifting

• Yoga

https://support.garmin.com/en-US/?faq=xQwjQjzUew4BF1GYcusE59#:~:text=The%20optical%20heart%20rate%20sensor%20must%20remain%20in%20contact%20with,can%20actually%20restrict%20blood%20flow. 

Boggy997 said:

My problem is if I don't pick a sport my Fenix 7 won't go above about bpm, but as soon as I remember and select a sport it reads more accurately.

As often mentioned in other threads on this topic, the HR sensor is controlled differently (sample rate). If you do not start an activity profile, the sensor is in a "power-saving mode" and is queried less frequently. This is one of the reasons why the watch has such a good battery life.
Normally, the sensor should go into the correct high-power mode when the accelerometer detects movement that indicates activity. However, this can be difficult with the sports linked in the support article above.

Simple solution :
When you do sports, start the corresponding activity profile!

If the sensor were to run 24/7 in high power mode, the fenix would only have a battery life like the Pixel watch. I don't think anyone wants that.

So start an activity profile ;)

Yes, I'm having this exact problem. 

Watch tight on wrist, riding indoor bike trainer, no workout selected, riding intervals of 105% FTP - Heart Rate readout is 105BPM  I wish!

Does anyone have a solution for this or is this a warranty issue with Garmin since it's a common occurrence? 

6772450 said:

Does anyone have a solution for this

Yes as noted in the post before yours. If you’re doing an activity then start an activity. The sampling rate is understandably reduced when not exercising as it can be expected that heart rate will change little in general and if it does it will be gradual. Next time you ride a bike indoors start an appropriate activity and you should see a difference. If you don’t see your heart rate being measured as expected then you might well have a problem in which case, contact Garmin Support. 

Many thanks Philip! Much appreciated :)

This is really bad and annoying bug that Garmin can't repair.....

6237469 said:

This is really bad and annoying bug

The problem being experienced will only be a bug if it occurs when the OP starts an activity and has the same outcome as before when he rode the bike indoors without starting an activity. 

It’s not rocket science. If you want accurate heart rate when exercising start an appropriate activity.