Navien Furnace E788 Error Code: Meaning & Safe Checks
Direct answer
What does E788 mean on a Navien furnace?
The official NPF Condensing Hydro-furnace Installation and Operation Manual identifies E788 as abnormal gas-type settings. The official table associates it with subcode 00. This meaning is limited to the documented NPF700 condensing hydro-furnace.
Search/display variants: E788, E788-00
Before you begin
Safety first
This record preserves the manufacturer-stated furnace fault meaning only. Do not open panels, test live voltage, handle refrigerant, adjust gas pressure, bypass a safety device, or replace a part from the code alone.
- Photograph the complete main code and any displayed subcode before cycling power.
- Confirm the full rating-label model against the models listed in the cited manual.
- Stop use for gas odor, smoke, sparking, overheating, water leakage, or repeated breaker trips.
Does this guide apply?
- Region
- United States, Canada
- Product family
- NPF700 condensing hydro-furnace
- Exact models
- NPF700-060U3BH, NPF700-080U3BH, NPF700-100U5CH, NPF700-060H3BH, NPF700-080H3BH, NPF700-100H5CH, NPF700-060D3BH, NPF700-080D3BH, NPF700-100D5CH
- Scope note
- The cited manual explicitly covers NPF700-060U3BH, NPF700-080U3BH, NPF700-100U5CH, NPF700-060H3BH, NPF700-080H3BH, NPF700-100H5CH, NPF700-060D3BH, NPF700-080D3BH, NPF700-100D5CH. Confirm the complete rating-label model before applying this definition.
Find the model label and official manual before ordering a part
Common paths to check
These are source-backed possibilities, not probability estimates for your specific unit.
- 01 Official manual meaning: Abnormal gas-type settings.
Decision path
Safe checks, in order
- 1
Confirm the full display and model
Record the main code and subcode exactly as displayed, photograph the rating label, and confirm that the cited manual covers the complete model.
Expected: The complete display and rating-label model match the official manual entry.
If not resolved: Contact Navien support with the model, serial number, full code, and cited manual link.
- 2
Keep inspection external and safe
Check only for visible unsafe conditions and unobstructed user-accessible intake, outlet, filter, drain, or vent areas described for the exact model; do not remove service panels.
Expected: No unsafe external condition is present and no internal access is required.
If not resolved: Stop using the furnace and arrange service by a technician qualified for this equipment.
Reset and test
A user reset is not documented for this condition.
Important: No universal reset is asserted. A repeated safety, combustion, pressure, electrical, refrigerant, or communication fault requires model-qualified diagnosis.
Escalation point
Stop and arrange service when
- There is a gas or refrigerant odor, smoke, sparking, overheating, water leakage, unusual pressure noise, or repeated breaker tripping.
- The complete model, main code, or subcode does not match the cited manual.
- The official procedure requires panel removal, live electrical testing, combustion work, gas-pressure work, refrigerant handling, or bypassing a safety device.
Model-first repair planning
Components that may be involved
The items below describe the documented diagnostic path. They are not a diagnosis and do not identify a compatible replacement part without the complete model and original part number.
- No replacement part identified without model-specific diagnosis
Official sources
We paraphrase manufacturer material and show the publication scope so you can verify it.
- NPF Condensing Hydro-furnace Installation and Operation Manual Navien · PDF page 128, E788 subcode 00 entry · accessed 2026-08-09
Reviewed by Device Fix Guide · Last source check:
Related guides for this equipment
These links share the same brand and equipment family. A related guide does not prove that the codes have the same cause on your model.
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