Direct answer: A Navien tankless water heater showing code E302 is usually dealing with low water pressure or poor water flow through the unit. The first things to check are whether the service valves are fully open, whether the cold-water inlet filter is packed with debris, and whether the house has weak incoming water pressure.
Most likely: Most often, the cold-water inlet screen is dirty, a valve is partly closed, or scale and debris are choking flow enough to trip the code.
Treat this like a flow problem first. Start outside the unit with the simple stuff you can see, then move to the inlet filter and basic flushing only if the heater has been running fine until recently. Reality check: a lot of E302 calls end up being a dirty inlet screen or a half-open valve. Common wrong move: clearing the code over and over without fixing the restriction that caused it.
Don’t start with: Do not start by ordering electronics or opening gas-side components. This code is usually a water-side problem, not a board problem.
The display shows E302 and the unit will not keep making hot water.
Start here: Start with the easy water-supply checks: confirm the cold-water supply valve and service valves are fully open and that house pressure is normal.
Cold taps feel normal, but hot taps are noticeably weaker or fall off after a few seconds.
Start here: Go straight to the water heater inlet side. A clogged cold-water inlet filter or scale restriction is more likely than an electrical problem.
The code showed up after a valve replacement, water outage, or work on the plumbing line.
Start here: Look for a partly reopened valve, debris knocked loose into the inlet filter, or air in the line.
You can reset the unit, but the code comes back when someone opens a hot tap.
Start here: That usually means the restriction is still there. Check flow through the unit instead of chasing the display.
This is the most common field find after water work, sediment movement, or long service intervals. The unit sees poor incoming flow and throws the code.
Quick check: Shut off water to the unit, remove the inlet filter if your setup allows safe access, and inspect for grit, scale flakes, or pipe debris.
A handle that looks open can still be left partly shut after maintenance. That cuts flow enough to trigger the code, especially when more than one fixture calls for hot water.
Quick check: Verify the cold-water supply valve and any service isolation valves at the heater are fully open, not just mostly open.
If pressure is weak at both hot and cold fixtures, the heater may be reacting to a supply problem upstream of the unit.
Quick check: Compare pressure at a cold tap and a hot tap. If both are weak, check the main shutoff, pressure-reducing valve issues, or recent municipal water problems.
On units that have not been flushed on schedule, mineral buildup can narrow the water path and cause recurring flow-related faults.
Quick check: If valves are open and the inlet filter is clean but hot flow is still poor, a descaling flush is the next likely step.
E302 is most useful when you separate weak-house-pressure problems from restrictions inside the tankless water heater.
Next move: If cold pressure is normal and hot pressure is clearly weaker, keep going on the water-heater side. If both hot and cold are weak throughout the house, the tankless unit may not be the main problem.
What to conclude: Normal cold with weak hot points toward a restriction at or inside the water heater. Weak pressure everywhere points toward house supply, a main valve issue, or another plumbing restriction.
Partly closed valves are common after service, and they can mimic a bigger failure.
Next move: If the code stays gone and hot water flow returns, the problem was a supply restriction from valve position. If the code returns during use, move to the inlet filter check.
What to conclude: A quick recovery after opening valves confirms the heater was being starved for water, not failing electronically.
This is the most likely hands-on fix when E302 shows up and cold pressure in the house is otherwise normal.
Next move: If hot water flow is back and the code does not return, the clogged inlet filter was the cause. If the filter was clean or the code comes back right away, the restriction is likely farther inside the unit or upstream in the plumbing.
Mineral scale can narrow the water path enough to trigger E302 even when the inlet filter looks fine.
Next move: If the code stays away and hot flow improves, scale restriction was likely the problem. If there is no improvement after a proper flush, stop short of deeper teardown and move to professional service.
A short, deliberate retest tells you whether the fix held and gives a technician useful information if it did not.
A good result: If the unit runs normally through both tests, keep an eye on flow over the next few days and plan regular maintenance.
If not: If the code still returns, the next step is professional diagnosis of internal water-side components or controls.
What to conclude: At that point you have ruled out the common homeowner fixes and avoided buying the wrong part.
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It usually means the unit is not seeing enough water pressure or flow through the heater. In real houses, that often comes down to a dirty inlet filter, a partly closed valve, low incoming pressure, or scale buildup restricting flow.
You can try one reset after checking valve position, but if the code comes back during use, the restriction is still there. Repeated resets do not fix the cause and usually just delay the real repair.
That points more toward a restriction at the tankless water heater than a whole-house pressure problem. The inlet filter and internal scale are the first places to look.
Not as a first move. This code is much more often caused by water-side flow trouble than by electronics. Rule out valves, the inlet filter, and scale before anyone starts guessing at internal controls.
Call for service if the code stays after you confirm valve position, clean the inlet filter, and perform a proper flush when due. Also call right away if you smell gas, find leaks, or are not comfortable isolating the unit safely.