A7 Error Code Troubleshooting Bosch
User Manual: A7 error code troubleshooting-Bosch
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Model: 250SX, 250SXO, 635ES, 635ESO, 2400E, 2400EO
A7 Error Code Troubleshooting
Service Bulletin: TWH-G2-31
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 A7 error indicates a problem related to the outlet hot water
temperature sensor. The sensor may be sensing temperatures
outside normal parameters or the sensor itself may be defective.
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. Verify the heater is not installed in a location that may be
exposed to freezing temperatures.
4. 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.
5. 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.
Hot sensor
Cold sensor
Hot sensor
Cold
senso
r
Figure 1
6. Inspect the hot water temperature sensor:
a. Sensor should be positioned on the top of the horizontal
section of the hot water pipe.
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.
f. If cleaning and repositioning the sensor fail to x the error, the
sensor may need to be replaced.
7. The inlet cold water sensor and the outlet hot water sensor are
identical. As a test, swap the hot and cold sensors.
a. Turn off the heater and disconnect both the cold and the hot
water sensors from the pipes.
b. Disconnect the blue wires from the cold water sensor.
c. Disconnect the red wires from the hot water sensor.
d. Swap the sensors, placing the cold water sensor on the hot
water side and vice versa, and reattach the wires to each sensor.
(Swap the sensors only, not the wires.)
e. After swapping the sensors, turn the heater on and try resetting
any error codes that that are displayed. If the error has now
changed from A7 to E2, the error is following the sensor,
indicating the sensor is likely defective and will need to be
replaced. (Part # 8700400015.)
f. If the E2 error continues after swapping the sensors and
resetting the error code, there may be a problem with
the wiring harness or control board. Contact Bosch Tech
Support.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
| TWH-G2-31 | 250SX, 250SXO, 635ES, 635ESO, 2400E, 2400EO Service bulletin
Bosch Thermotechnology Corp.
Data subject to change without notice | Printed in the USA | BTC 711002311 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
8. For an alternate test to check the sensors with a multimeter, refer
to Bulletin TWH-G2-05 for testing procedure.
