Bosch E3 Error Code — What It Means
On Bosch Greentherm tankless water heaters, the E3 (or A3) error indicates the flue gas temperature sensor is disconnected, damaged, or that flue gas temperature has exceeded the trip threshold for too long. The unit will report this fault when flue gases rise above 145°F for two minutes or when the sensor circuit is open. The code protects the heater by shutting down operation until the condition is corrected.
Common Causes
- Disconnected or loose sensor wiring The flue gas temperature sensor harness or connector has come loose or is open, breaking the circuit.
- Faulty exhaust temperature sensor The flue gas temperature sensor itself has failed and no longer reads correctly or matches the resistance chart.
- Restricted venting or flue blockage Obstructed exhaust pipes or poor ventilation installation can trap heat and drive flue gas temperature above the trip point.
- Scale buildup in the heat exchanger Mineral deposits restrict water flow through the exchanger, reducing heat transfer and causing the exhaust gases to overheat.
- Low water flow through the unit Clogged inlet screens, closed isolation valves, or low system pressure can reduce flow and allow the exchanger to run hot.
- Damaged sensor wiring or connector corrosion Corroded terminals or pinched wires in the sensor harness can create intermittent opens that the control board reads as a fault.
Step-by-Step Fix
- Turn off power at the breaker and close the gas supply valve to the heater before any inspection or service work.
- Locate the flue gas temperature sensor on the exhaust side of the unit and inspect the wiring harness and connector for damage, corrosion, or loose pins.
- Reconnect or reseat the sensor connector if it is loose or disconnected, then restore power and test for a reset of the fault.
- Test the sensor resistance with a multimeter if the wiring is intact, and compare the reading at ambient temperature to the resistance chart in your model’s service manual.
- Replace the flue gas temperature sensor if the resistance reading is out of specification or the sensor is visibly damaged.
- Flush and descale the heat exchanger using a tankless flush kit and manufacturer-approved descaling solution if the sensor tests good but the unit is genuinely overheating.
- Inspect the venting system for restrictions, blockages, or installation errors that could trap heat, and correct any issues found to bring exhaust temperature back into range.
Parts Often Needed
| Part | Notes |
|---|---|
| Bosch flue gas temperature sensor | Amazon | Match the sensor to your specific Bosch tankless model and serial number for correct fit. |
| Sensor wiring harness or connector | Amazon | Order if terminals are corroded or the harness insulation is damaged beyond repair. |
| Tankless water heater flush kit | Amazon | Includes pump, hoses, and descaling solution for heat exchanger maintenance and scale removal. |
When to Call a Pro
Call a licensed plumber or gas technician if you are not comfortable working with gas appliances, testing electrical sensors with a meter, or isolating and flushing the heat exchanger. A professional should handle sensor replacement if the fault persists after checking wiring, and any venting work that involves modifying exhaust pipes or correcting installation code violations. If the unit continues to overheat after descaling and sensor replacement, a technician can diagnose deeper control board or gas valve issues that require specialized tools and training.