Bosch Tankless A4 Error - Causes & Fix

Bosch Tankless A4 Error — What It Means

The A4 error code on a Bosch tankless water heater indicates a backflow temperature sensor defect. This sensor monitors the temperature of incoming water at the inlet pipe. The error trips when the sensor is disconnected, has a short circuit, or is electrically faulty (open circuit).

The backflow sensor is typically located on the lower-left corner of the unit, clipped to the horizontal copper inlet pipe. Two red wires connect it to the control board. When the sensor cannot send a valid signal, the unit shuts down to protect itself and displays A4.

Before You Replace Anything

Homeowners sometimes replace the sensor itself when the real problem is a loose wire connector or poor contact between the sensor clip and the copper pipe. Always check connections and sensor seating before ordering a new sensor.

Jump to Fix

Common Causes

• Loose or damaged sensor wiring (~40%) The two red wires at the backflow temperature sensor connector may be loose, disconnected, or have damaged insulation causing a short or open circuit.
• Poor sensor contact with pipe (~25%) The sensor clip may not be making good contact with the copper backflow inlet pipe, leading to erratic or no readings.
• Defective temperature sensor (~20%) The sensor itself may have failed internally (open or shorted) and no longer produces a valid resistance reading.
• Exposure to freezing conditions (~10%) If inlet water temperature drops below 2°C (35.6°F), the sensor may trip and trigger the error, requiring freeze protection.
• Flue gas limiter or fuse wire issue (~5%) On older models, a defective or disconnected flue gas limiter (fuse wire) can sometimes cause related sensor faults.

Quick Diagnosis

Answer these to narrow it down fast.

Step-by-Step Fix

1. Turn off power at the breaker for 5-10 minutes to reset the control board and clear transient faults.
2. Restore power and verify gas and water supply: open the gas valve fully and check that water lines are unobstructed.
3. Inspect venting for blockages in the exhaust and air intake pipes, and confirm adequate combustion air supply.
4. Locate the backflow temperature sensor on the inlet (cold water) copper pipe, typically in the lower-left corner of the unit inside the case.
5. Check sensor seating: verify the sensor clip is firmly attached to the middle of the horizontal section of the copper pipe, not at a joint or elbow.
6. Inspect the two red wires: make sure they are securely connected at both the sensor terminal and the connector plug on the control board harness, with no damaged insulation.
7. Measure sensor resistance (if equipped with a multimeter): disconnect the sensor, measure resistance across its terminals, and compare to the manufacturer’s specification (typically 10kΩ at 25°C for standard Bosch NTC sensors, though consult your model’s datasheet).
8. Replace the sensor if resistance is 0Ω (short) or infinite (open), or if connections are secure but the fault persists.
9. Reset the error code: hold the ON/OFF button for 3 seconds on touch models, or unplug the unit for 2-3 minutes on older models, then verify normal operation.

Parts Often Needed

When to Call a Pro

Call a licensed plumber or gas technician if you are uncomfortable working with gas appliances, if the unit requires disassembly to access internal components, or if the error persists after you have checked wiring and reseated the sensor. Tankless water heaters involve both gas and electrical systems. A qualified technician can safely measure sensor resistance, diagnose control board faults, verify proper grounding (120V AC, 60Hz), and check gas pressure. If the inlet water is exposed to freezing conditions, a pro can install freeze protection or relocate the unit. Always follow local codes and manufacturer guidelines when servicing gas-fired equipment.

Rough cost: A pro service call runs about $150-300.