Thermocouple
Common pilot and ignition part on gas water heaters.
Navien E302 Error Code — What It Means
The Navien E302 error code means low system water pressure. On Navien combi-boilers and hydronic heating systems, E302 trips when the system water pressure sensor reads below the minimum threshold — typically below 7–10 PSI depending on the model. The unit shuts down heating operation to prevent running the heat exchanger dry. E302 is one of the most common Navien service calls and in most cases is resolved by repressurizing the system, not replacing any parts.
Common Causes
- Water loss from a leak — A slow leak anywhere in the hydronic loop (expansion tank, air vent, baseboards, or fittings) gradually drops system pressure until E302 triggers.
- Waterlogged or failed expansion tank — If the expansion tank bladder has failed, pressure swings wildly and eventually drops below the threshold under operating conditions.
- Auto-fill valve closed or failed — Systems with an automatic fill valve maintain pressure automatically; if the valve is closed or stuck, pressure won’t be maintained.
- Air in the system — Trapped air creates a false low-pressure condition; purging the system restores accurate pressure readings.
Step-by-Step Fix
- Check the system pressure gauge — Look at the analog pressure gauge on the unit or piping. Normal cold-fill pressure is 12–15 PSI for most residential systems. If below 10 PSI, the system needs water added.
- Repressurize using the fill valve — Locate the manual fill valve on the cold-water makeup line feeding the boiler loop. Slowly open the valve until the gauge reads 12–15 PSI, then close it.
- Inspect for visible leaks — Walk the entire piping loop looking for wet spots, stains, or dripping at joints, air vents, and radiator valves. Even a slow weep will cause E302 to recur.
- Test the expansion tank — Shut off the boiler and close the isolation valve, then check the expansion tank’s Schrader valve with a tire gauge. Should read 12 PSI cold. If water comes out, the bladder is failed and the tank needs replacement.
- Reset the system — Press the RESET button on the Navien unit. If pressure holds and E302 clears, monitor the gauge over 24 hours. Recurrence = there’s a leak you haven’t found yet.
Parts Often Needed
| Part | Notes |
|---|---|
| Expansion tank (2–4 gallon for residential) | Replace when bladder fails; size to system volume |
| Manual fill valve | Replace if valve body is corroded or won’t fully open/close |
| Air vent (automatic Caleffi or Taco) | Replace leaking auto-vents on high points of the loop |
Need Replacement Parts?
Before ordering, match the water-heating part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
If you repressurize and E302 returns within hours, there’s a leak somewhere in the system that requires pressure testing to locate. A plumber or boiler tech can perform a static pressure test to pinpoint the leak before it causes heat exchanger damage.