What does Bosch E4 Error Code - Causes & Fix mean?

E4 means backflow temperature over 309°F. Most often caused by blocked venting or a failed secondary fan. Check vent and fan first.

How do I fix Bosch E4 Error Code - Causes & Fix?

Follow the step-by-step diagnosis below. The most likely fix is identified by checking power, sensor resistance, and control-board signals in order — using a multimeter and the part numbers documented in the parts reference table.

Can I keep running the equipment with E4 active?

Generally no. Faults on Water heater units are emitted when a sensor, safety, or control circuit is outside its operating envelope. Running through an active fault risks compressor, heater, or control-board damage. Diagnose and clear the fault first.

What part is most commonly replaced for Bosch E4 Error Code - Causes & Fix?

The most frequently replaced parts for Bosch E4 Error Code - Causes & Fix are listed in the Parts Reference section of this guide with OEM part numbers and typical price ranges. Check the table for direct replacements.

Is Bosch E4 Error Code - Causes & Fix a DIY repair or do I need a technician?

Sensor swaps, fan motor replacements, and condenser cleaning are typical DIY-level repairs. Anything involving refrigerant, gas, or high-voltage (>240V) work requires a licensed technician for both safety and code-compliance reasons.

Bosch E4 Error Code - Causes & Fix

Bosch E4 Error Code — What It Means

The E4 error on a Bosch tankless water heater indicates that the backflow temperature sensor has detected a temperature above 309°F, triggering a safety shutdown to prevent damage or fire risk. This overheating fault tells you the appliance is trapping too much heat, either because hot exhaust gases are not venting properly or because the combustion air supply and fan system are compromised.

The root cause is almost always related to airflow: blocked or improperly installed venting, a failed or obstructed secondary fan, or poor combustion-air conditions. Less commonly, the sensor itself or its wiring has failed and is sending a false high-temperature signal to the control board.

Jump to Fix

Common Causes

Blocked or restricted venting Exhaust or intake pipes that are obstructed, kinked, or installed with excessive length or too many elbows prevent proper airflow and cause internal overheating.
Failed or obstructed secondary fan The secondary fan may have stopped spinning due to a burned-out motor, damaged bushings, debris in the blades, or wiring faults, cutting off the airflow needed to cool the heat exchanger.
Faulty backflow temperature sensor or wiring The sensor itself may have drifted out of calibration, or its connector and harness may be loose, corroded, or damaged, sending false overheat signals to the control board.
Poor combustion-air conditions or vent leakage If the appliance is installed in a confined space without adequate fresh air or if vent joints are leaking exhaust back into the combustion chamber, the unit will overheat and trip the sensor.

Step-by-Step Fix

Verify the lockout and reset the unit. Confirm that the display shows E4, then turn off power to the water heater for 30 seconds and restore power to see if the fault recurs immediately or only under load.
Inspect the vent system for obstructions and compliance. Check both the exhaust and intake terminations outside for blockages (leaves, ice, bird nests), then verify that the total vent length and number of elbows match the limits in your Bosch installation manual for your model and vent diameter.
Examine the backflow temperature sensor and its connections. Locate the sensor on the return pipe near the heat exchanger, unplug the connector, inspect the pins and socket for corrosion or damage, and reseat firmly.
Test the secondary fan operation. With the unit firing and burners lit, listen for fan noise and verify the fan is spinning freely. If Bosch troubleshooting documents for your model specify measuring voltage between the two black wires on the secondary fan, use a multimeter to confirm voltage is present when the burners are on.
Check for fan circuit voltage and wiring integrity. If the fan is not running but voltage is present at the fan connector, inspect the fan motor bushings and blades for binding or debris and remove any obstructions. If voltage is absent, trace the wiring harness back to the control board for breaks or loose connections.
Replace the backflow temperature sensor if venting and fan checks pass. If all airflow components are confirmed good and the fault persists, the sensor is likely reading high due to internal failure and should be replaced with a genuine Bosch part.
Consult a licensed technician if the fault remains after sensor replacement. If the new sensor still triggers E4 and all vent and fan diagnostics are normal, the control board may have a circuit fault requiring professional diagnosis and replacement.

Parts Often Needed

Part	Notes
Bosch backflow temperature sensor	Amazon \| Match the sensor part number to your specific Bosch tankless model (Greentherm, Therm, AquaStar series).
Bosch secondary fan motor assembly	Amazon \| Order the correct fan assembly by your model number if the motor is noisy, won’t spin, or has damaged bushings.
Wiring harness for temperature sensor	Amazon \| Use an OEM Bosch harness if the original connector or wiring is corroded, melted, or broken.

When to Call a Pro

Call a licensed plumber or gas-appliance technician if you are uncomfortable working with gas connections, electrical diagnostics, or vent disassembly. Professional help is also necessary if the E4 fault persists after you have verified vent compliance, confirmed the secondary fan is running and receiving voltage, and replaced the backflow temperature sensor. Overheating faults can indicate subtle combustion or control-board issues that require specialized test equipment and training to diagnose safely.