Navien Water Heater Error Code 598-00 - A Pro'S Guide To Diagnosing

That sinking feeling. You’re ready for a hot shower, but the water stays stubbornly cold. A quick look at your Navien tankless water heater reveals a blinking light and a cryptic message: Navien water heater error code 598-00. It’s a frustrating moment that can throw your whole day off track.

I get it. As an HVACR technician, I’ve seen that look of confusion on countless homeowners’ faces. But here’s the good news: you’ve come to the right place. We’re going to demystify this error code together, step by step.

This comprehensive guide will promise to do more than just tell you what the code means. We’ll walk you through what’s happening inside your unit, show you a few safe and simple checks you can perform yourself, and help you understand exactly when it’s time to call for professional backup. Let’s get that hot water flowing again.

What Exactly is Navien Water Heater Error Code 598-00?

First things first, let’s translate the technical jargon. In the official Navien manual, error code 598-00 points to a “Flow Sensor Malfunction.”

Think of the flow sensor as the gatekeeper for your hot water. It’s a small but vital component that detects when you turn on a hot water tap anywhere in your house. Its one job is to send a signal to the main control board—the unit’s brain—saying, “Hey, we need hot water now! Fire up the burner!”

When this sensor malfunctions, the signal never gets sent. Your Navien heater remains idle, completely unaware that you’re waiting for it to do its job. Essentially, the communication link is broken, and without that “go” signal, you get no hot water.

Uncovering the Common Problems Behind Error Code 598-00

While the error points to the flow sensor, the sensor itself isn’t always the broken part. Several issues can cause the system to think the sensor has failed. Understanding these common problems is the key to a quick diagnosis. This is the perfect starting point for our navien water heater error code 598-00 guide.

A Stuck or Failed Flow Sensor

This is the most direct cause. Over time, hard water minerals (limescale) and tiny bits of sediment can build up inside the sensor assembly. This buildup can cause the sensor’s internal paddle or turbine to get stuck, preventing it from registering water flow.

In other cases, the electronic component of the sensor can simply wear out and fail, requiring a replacement.

Low Water Flow or Pressure

Sometimes the sensor is working perfectly, but the water flow through the unit is too low to trigger it. Navien heaters have a minimum flow rate required for activation (usually around 0.5 gallons per minute).

If the flow drops below this threshold, the sensor won’t activate, and the unit will throw the 598-00 error. This can be caused by:

A clogged faucet aerator or showerhead
A water-saving fixture that doesn’t draw enough water
A main water valve to the house or heater that is not fully open

Debris in the Water Inlet Filter

This is one of the most common and easily fixable culprits. Your Navien heater has a small, mesh filter screen on the cold water inlet pipe to protect the internal components from debris.

If this filter becomes clogged with sediment, rust, or sand, it severely restricts the amount of water entering the unit. The result is low flow, which, as we just learned, fails to trigger the flow sensor.

Wiring and Connection Issues

The flow sensor communicates with the main control board via a set of wires. On rare occasions, these wires can become loose, corroded (especially in damp basements), or even damaged by pests.

A poor connection means the signal from a perfectly healthy sensor never reaches the control board, leading to the 598-00 error code.

A Failing Main Control Board (PCB)

This is the least likely scenario, but it’s a possibility. The Printed Circuit Board (PCB) is the heater’s brain. It could be that the flow sensor is sending a clear signal, but the board is unable to receive or interpret it correctly.

Diagnosing a faulty PCB is a job strictly for a qualified technician, as it involves testing electronic components.

Your Step-by-Step DIY Troubleshooting Guide for Error 598-00

Alright, let’s roll up our sleeves. Before you pick up the phone to call for service, there are a few safe and simple checks you can perform. Don’t worry—we’ll walk through these together. Always prioritize your safety; if you feel unsure about any step, it’s best to stop and call a professional.

Step 1: The Simple Reset (Power Cycle)

Electronics can sometimes experience temporary glitches. The first and easiest step is to perform a power cycle.

Locate the power cord for your Navien unit and unplug it from the wall outlet.
Wait for at least one full minute. This allows the internal capacitors to discharge and the control board to fully reset.
Plug the unit back in.
Turn on a hot water tap and see if the error code returns. If it doesn’t, you may have just fixed a temporary glitch!

Step 2: Check Your Faucets and Fixtures

Next, let’s rule out a simple low-flow issue at a single fixture. Turn on the hot water at several different faucets in your home—the kitchen sink, a bathroom sink, and the bathtub.

If the heater fires up for the bathtub faucet (a high-flow fixture) but not for a bathroom sink, the problem is almost certainly a clogged aerator on that specific sink. Simply unscrew the tip of the faucet, clean the small screen inside, and screw it back on.

Step 3: Inspect the Water Inlet Filter

This is the most hands-on step, but it’s also the most likely solution. If you’re comfortable with a few basic tools, this is a great way to solve common problems with navien water heater error code 598-00.

Turn Off the Water: Locate the cold water shut-off valve leading to your Navien unit. It will be on the right-side pipe and usually has a blue handle. Turn the handle so it is perpendicular to the pipe.
Turn Off the Power: Unplug the unit from the wall outlet for safety.
Relieve Pressure: Open a nearby hot water tap to let any remaining pressure and water drain out.
Locate and Remove the Filter: At the bottom of the cold water inlet connection, you’ll see a fitting. Use a pair of pliers or a small wrench to carefully unscrew it. Inside, you will find a small, thimble-shaped mesh screen. You may need to gently pry it out with a small flathead screwdriver.
Clean the Filter: Hold the filter under a faucet and use an old toothbrush or a soft brush to gently scrub away all the sediment and debris. It should look clean and clear when you’re done.
Reassemble: Carefully place the clean filter back into the fitting, and screw the fitting back onto the unit. Hand-tighten first, then give it a gentle quarter-turn with your pliers. Do not overtighten.
Restore Water and Power: Slowly turn the water valve back on. Check for any leaks. Plug the unit back in and test your hot water.

When to Put Down the Tools and Call a Navien Pro

Knowing your limits is the hallmark of a smart homeowner. If you’ve tried the DIY steps above and the navien water heater error code 598-00 persists, it’s time to call a qualified technician.

Here are the clear signs you need professional help:

The error code comes back immediately after a reset and cleaning the filter.
You are not comfortable shutting off the water or opening any fittings on your unit.
You see signs of water leakage around the flow sensor assembly or other internal parts.
The problem likely requires testing electrical connections or replacing the flow sensor itself.

We strongly recommend hiring a technician who is certified by Navien. They have the specific training, diagnostic tools, and access to genuine parts to fix the issue correctly and efficiently, ensuring your warranty remains intact.

Best Practices for Preventing Navien Error Codes: A Care Guide

The best way to deal with an error code is to prevent it from ever happening. Following a simple navien water heater error code 598-00 care guide can dramatically improve your unit’s reliability and lifespan. Adopting these habits is also an eco-friendly navien water heater error code 598-00 strategy, as it ensures your heater runs at peak efficiency, saving energy.

Regular Descaling and Flushing

For homes with hard water, this is the single most important maintenance task. Limescale is the number one enemy of tankless water heaters. Performing an annual (or semi-annual in very hard water areas) descale flush dissolves the mineral buildup that causes flow sensors to stick and heat exchangers to lose efficiency. This is a key part of any sustainable navien water heater error code 598-00 prevention plan.

Annual Professional Maintenance

Having a professional service your unit once a year is a fantastic investment. During a tune-up, a technician will clean the inlet filter, inspect the burner assembly, check all wiring, and test sensor operations. They can spot and fix small issues before they become error-code-inducing problems.

Frequently Asked Questions About Navien Water Heater Error Code 598-00

Can I clear error code 598-00 myself?

Yes, in many cases. You can often resolve the error by performing a power cycle (unplugging the unit for a minute) or by cleaning the cold water inlet filter. If these simple steps don’t work, the issue is likely internal and requires a professional.

How much does it cost to fix a Navien flow sensor?

The cost can vary depending on your location and the technician’s labor rates. The flow sensor part itself is relatively inexpensive. Generally, you can expect a service call fee plus the cost of the part and about an hour of labor. It is a much less costly repair than replacing a heat exchanger or main control board.

Is it safe to keep using my water heater with error 598-00?

The unit will likely not produce any hot water while this error is active, so you won’t be able to use it for that purpose. The code itself does not indicate a dangerous condition like a gas leak or overheating. However, it indicates a functional failure that needs to be addressed to restore your hot water supply.

Tackling an error code on a sophisticated piece of equipment like a Navien tankless heater can feel daunting, but you’re now armed with the knowledge to understand what’s happening. By following the safe DIY checks and knowing when to call for expert help, you’ve taken control of the situation.

Remember, regular maintenance is the key to a long and trouble-free life for your water heater. Stay safe, and here’s to getting that wonderful hot water flowing again!

Author
Recent Posts

David Clark

Founder & Lead HVAC Specialist at HVACRLY

David Clark is the founder of HVACrly and a certified HVAC specialist with 10+ years working on residential and light-commercial systems. He turns complex heating, cooling, ventilation, and refrigeration topics into clear, actionable guides focused on safety and energy efficiency. When he’s not on a service call, he’s testing tools and writing step-by-step tutorials.