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Troubleshooting
6.8 Error code diagnostics
Part numbers subject to change.
Display Cause Solution
A2 Fault in the flue gas limiter (part#
8700400032).
Note: Old style overheat cutoff fuse
(8700400019) has been replaced with this
new limiter.
1. Check for flue gas leakage around the top and
bottom seals of heat exchanger as well as the
viewing window. Look for any noticeable burn marks
or warping of the gaskets. If a leak is detected, seals
will need replacing.
2. Verify that venting meets specifications outlined
in chapter
3.6. Long vent lengths, incorrect venting
material, venting with more than three elbows,
blocked vent or combination venting may cause this
failure. Verify backdraft flapper inside flue gas
collector is not stuck shut.
3. Check flue gas limiter wire connectors for
tightness. Clean with pencil eraser if any corrosion
is evident.
4. Trace wires from flue gas limiter to black
connector. Verify the connection is tight.
5. If limiter is tripped due to overheating, it must cool
down before error will clear on the control board.
A7 Hot water temperature sensor fault
(Temperature below 36°F or above 210°F)
(part# 8700400015)
1. Check red wire connections at the hot water
temperature sensor. Clean terminals with a pencil
eraser. If badly corroded, replace sensor and wire
harness. Verify that sensor is firmly clipped on to the
hot water copper pipe.
2. Sensor may trip if water temperature drops below
36°F. Protect heater from freezing condition as any
damage due to freezing conditions is not covered
under warranty.
3. Clean inlet filter screen per chapter
5.1 and clear
any debris out of showerheads, faucet aerators and
whole house filters.
4. In areas where water has a high mineral content,
periodic descaling may necessary. See chapter
5.3
for directions.
5. Swap sensor with the cold water sensor, keeping
respective wire connections as they are. If the
heater produces an E2 error, replace the sensor.
A9
(Flashing)
Hot water temperature sensor not sensing
expected output temperature.
(part# 8700400015)
1. Check that the sensor is firmly attached to the
horizontal section of the hot water pipe.
2. Clean inlet filter screen per chapter
5.2 and clear
any debris out of showerheads, faucet aerators and
whole house filters.
3. Verify that venting meets specifications outlined
in chapter
3.6. Long vent lengths, incorrect venting
material, venting with more than three elbows,
blocked vent or combination venting may cause this
failure. Verify backdraft flapper inside flue gas
collector is not stuck shut.
4. Check gas pressure per chapter
3.8*. Low gas
pressure may prevent the heater from reaching
desired output temperature.
5. Check supply voltage is 120VAC and is properly
grounded.
6. Possible defective control unit. Call Bosch Water
Heating for further instructions.
Table 12
* By installer or service technician only.