Bosch Tankless A9 Error — What It Means
The A9 error code indicates that the hot water temperature sensor is not detecting the expected outlet temperature. Bosch interprets this as incorrect output power that is too low. The sensor, which clips onto the horizontal section of the ½-inch copper hot water pipe on the lower left side of the unit, is reporting a temperature lower than the set point. The heater may continue to run in a safe mode while flashing the error, or it may shut off entirely and require a manual reset.
Before You Replace Anything
Many people replace the sensor when the actual problem is a loose clip or corroded spade connector. Clean the spade terminals with sandpaper and reseat the sensor firmly before ordering a new part.
Common Causes
- Sensor not making good contact with pipe (~40%) Sensor clip is loose or positioned incorrectly (should be on top of horizontal pipe section), preventing accurate temperature reading.
- Loose or corroded red wires (~25%) Two red wires connecting to sensor may be loose, corroded, or have dirty spade terminals blocking signal.
- Defective temperature sensor (~15%) Sensor itself is faulty and not reading accurately, though this is less common than physical contact issues.
- High inlet water temperature or low flow rate (~10%) Preheated water fed to unit or low flow prevents proper heating cycle, causing sensor to read lower temp than expected.
- Low gas pressure (~7%) Unit cannot reach set temperature due to insufficient fuel supply at burner.
- Blocked or improper venting (~3%) Poor combustion inhibits heating, or back-draft flapper may be stuck, reducing heat output.
Quick Diagnosis
Answer these to narrow it down fast.
Is the sensor clipped firmly on top of the horizontal copper pipe section?
No: Reposition sensor clip firmly on top of horizontal pipe. Reset unit and test for error.
Are the two red sensor wires and spade connectors clean and tight?
No: Clean spade terminals with fine sandpaper, reconnect firmly, reset unit and test.
Does the error persist after reseating sensor and cleaning connections?
No: Problem solved. Monitor for recurring error over next few uses.
Step-by-Step Fix
- Turn off power and gas to the heater at the service switches and gas valve.
- Remove front cover following the instructions in your owner’s manual.
- Locate the temperature sensor on the ½-inch copper hot water pipe on the lower left side of the unit.
- Inspect sensor clip position and verify it is firmly clipped on top of the horizontal pipe section, not underneath or to the side.
- Disconnect the two red wires from the sensor and examine the spade connectors and wire ends for corrosion or dirt.
- Clean spade terminals with fine sandpaper or emery cloth if any corrosion or oxidation is visible, then reconnect firmly.
- Check inlet filter screen for dirt or debris and clean if necessary (see Service Bulletin TWH G2-02).
- Verify cold water supply to confirm no preheated water is being fed to the unit from another source.
- Restore power and gas, reset the unit per your manual, and run hot water to test for error.
- If error persists, measure sensor resistance with an ohmmeter at a known temperature (example: approximately 3 ohms at 140°F) and compare to your model’s chart.
- Replace temperature sensor if resistance readings do not match or if physical inspection and cleaning did not resolve the error.
- Check gas pressure with a manometer if sensor replacement does not fix the problem, comparing inlet and manifold pressure to nameplate values.
Parts Often Needed
| Part | Notes |
|---|---|
| Bosch tankless water heater outlet temperature sensor | Amazon | Verify part number for your specific Bosch model before ordering. |
| Inlet filter screen kit | Amazon | If screen is damaged or heavily fouled and cannot be cleaned. |
When to Call a Pro
Call a licensed technician if you are uncomfortable working with gas connections, if you do not have a manometer to test gas pressure, or if the error persists after replacing the sensor and cleaning all connections. A technician can measure inlet and manifold gas pressure, inspect venting and combustion air supply, test supply voltage to the fan, and diagnose control board faults. If the unit requires adjustment to gas pressure or replacement of the control unit, professional service is required.
Rough cost: DIY runs about $30-60 in parts, 20-40 min. A pro service call runs about $150-300.