Bosch AE12 Troubleshooting Manual page 3

Checking the Thermal Cut Out:

1. Locate the thermal cut out switch (see Fig. 3).
Reset button on Thermal Cut Out
2. Press the button located in the center of the cut out
switch. If you feel it click (similar to a ball point pen), then
the thermal cut out was tripped.
NOTICE:
The thermal cut out trips because of another underlying
problem, such as scale, low fl ow with warm inlet water,
high temperature setting or preheated water. It is
important to fi nd the issue before returning the heater to
service.
3. Document your fi ndings in the Water Heater questionnaire
on page 6.
Step 4 – Checking continuity and resistance
Ensure the circuit breaker powering the appliance is still shut

off.
 Check the thermal cut out switch for continuity.
1. Check for continuity between connections labeled A-B and
also C-D (see Fig. 4).
2. Document your fi ndings in the Water Heater questionnaire
on page 6.
Data subject to change without notice | Printed in the USA | BTC 721002308 A | 09.2013
Figure 3
A
C
 Check the fl ow switch for continuity.
1. With no water fl owing through the unit, check continuity
between the brown wire and the yellow wire on the circuit
board using 100 ohms scale (see Fig. 5). A stuck or dama-
ged fl ow switch will show a closed circuit without water
fl ow causing the TCO to trip.
2. Now with water fl owing through the unit, check continuity
between the brown wire and the yellow wire on the circuit
board using 100 ohms scale (see Fig. 5).
3. Document your fi ndings in the Water Heater questionnaire
on page 6.
Troubleshooting Guide | AE12, RP12T, US12
| 3
Figure 4
B
D
Figure 5
Bosch Thermotechnology Corp.