A9 Error Code Troubleshooting Bosch
User Manual: A9 error code troubleshooting-Bosch
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Model: 250SX, 250SXO, 635ES, 635ESO, 2400E, 2400EO
A9 Error Code Troubleshooting
Service Bulletin: TWH-G2-32
Data subject to change without notice | Printed in the USA | BTC 711002312 A | 01.2010
Bosch Thermotechnology Corp.
50 Wentworth Avenue
Londonderry, NH 03053
Tel: 1-866-642-3198
Fax: 1-603-584-1681
www.boschpro.com
Introduction
Follow the procedures below and report results to Bosch Technical
Support. This will assist in determining the cause and solution to
the problem.
An A9 error indicates that the hot water temperature sensor is not
sensing the expected outlet temperature. The heater may continue
to run in a “safe” mode with the A9 error code ashing or the error
code may shut down the heater until the code is reset.
NTC Temperature Sensors
• The sensor is a negative temperature coefcient (NTC)
sensor that reads different temperatures based on different
resistances.
• Lower resistances are interpreted as higher temperatures;
higher resistances are interpreted as lower temperatures.
• See Bulletin TWH-G2-05 for approximate temperature sensor
resistance readings
Tools needed:
• Ohmmeter/ multimeter
• Phillips head screwdriver
• Fine sandpaper or emery cloth
Procedure
1. Press the reset button on the control panel of the heater to
attempt to clear the error code. If the error code does not
reset or continues to reoccur, proceed with the following
troubleshooting steps.
2. Verify the heater is being used for domestic hot water only and is
not being fed any preheated water.
3. Remove the front cover of the water heater. For instructions on
removing the front cover, refer to the installation/owner’s manual
of the water heater.
4. Locate the hot water temperature sensor on the 1/2” copper
pipe inside the lower left side of the water heater. The hot water
temperature sensor is connected with two red wires and should
be clipped to the horizontal section of the hot water pipe. See
Figure 1.
5. Inspect the hot water temperature sensor:
a. Sensor should be positioned on the top of the horizontal
section of the hot water pipe.
Hot sensor
Cold sensor
Hot sensor
Cold
senso
r
Figure 1
b. The sensor should not be positioned on an elbow or near the
bypass tube on the vertical section of the hot water pipe.
c. Unclip the sensor from the copper pipe and inspect the inside
of the sensor. Sensor should be clean and free of corrosion.
d. Sensor spade connections (red wires) must be clean and tight.
e. If wire connections, sensor, or pipe are corroded, clean with
ne sandpaper or emery cloth.
6. A9 error codes may be caused by a combination of high inlet
water temperatures and low water ow rates. If errors occur
only while running low ow xtures, increasing water ow may
resolve A9 error codes.
7. If error codes persist, contact Bosch Tech Support. To assist
tech support in determining the cause of the error code, please
document information from the following procedure:
a. With the heater on standby and the error code reset, turn
the on/off seconds, then turn the switch to the on (I)
position.
b. Quickly observe the series of letter and numbers that
appear on the LCD display screen. This series will typically
start with “SU” or “AF” followed by four digits. (Examples:
“SU 20 18”, “AF 30 05”). If necessary, repeat previous step
to conrm numbers. Record these letters and numbers for
reference by Bosch Tech Support.
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