http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
Service Bulletin G3-16
Models: 2400ES, 2700ES, 715ES, C800ES, C920ES, C920ESC, Integra 500, Evolution 500
A4 - E4 Error Code Troubleshooting
Introduction
The following procedures are for gathering information to assist
Bosch Tech Support in determining the cause and solution to the
problem. Unplug power cord and remove outer and inner covers
from heater (refer to Installation Manual).
a. Remove 3 screws attaching control unit to heater
(Fig.2, pos.1).
b. Turn control unit over (Fig.2, pos.2).
c. Remove 6 screws on rear cover of control unit (Fig.2, pos.3).
Figure 2
Error code explanation:
A4 error code explanation: Backflow temperature sensor fault
E4 error code explanation: Backflow temperature sensor over 309° F.
Tools needed:


Phillips head screwdriver
Multimeter
control unit front
A4 troubleshooting procedure
1. Check connection between backflow temperature sensor and
wiring harness. Black wires and connector should be plugged
into white connector with green wires (Figure 1).
Figure 1
black
connector
white
connector
d. Remove rear cover.
e. Locate wire connectors on edge of circuit board.
f. Remove and reinstall connectors onto edge of board to
improve the electrical connection (Fig.3, pos.1,2,3)
Figure 3
1
2. Remove control unit from heater and reseat wire harness
connector on circuit board:
2
3
4 | G3-16 | 2400ES, 2700ES, 715ES, C800ES, C920ES, C920ESC, Integra 500, Evolution 500
E4 troubleshooting procedure
1. Check intake, exhaust vent and terminations for:
a. Blockage or restriction.
b. Excess vent length over maximum. See manual for correct
vent requirements.
c. More than 4 90 degree elbows plus termination in each vent.
2. When heater is in operation with burners fired and covers off,
measure voltage between 2 black wires on secondary fan
(Fig.4, pos.1).
Service bulletin
c. Check fan blades for free spinning:
•
If debris is blocking blades in housing, remove fan and
debris, reinstall fan.
•
If blades hit housing, fan motor bushings may be
damaged and fan may need to be replaced.
Figure 5
base of burners
CAUTION!
Keep hands and tools away from spinning blades on right
side of fan.
1
Figure 4
2
1
Secondary fan
a. If there is no voltage present, wiring may be faulty: Ensure
that 3 wire connections to fan are secure.
3. If voltage is present but fan is noisy or does not spin: Turn off
power to heater and check the following:
a. Ensure that fan is securely mounted with 2 screws at top of
housing to underside of burner. (Fig. 5, pos. 1)
b. Check fan motor mounting bushings for damage. If damaged,
replace fan. (Fig. 5, pos. 2).
Note: 3rd bushing not visible in Figure 5. It is located toward
the rear of the fan.
Bosch Thermotechnology Corp.
50 Wentworth Avenue
Londonderry, NH 03053
Tel: 1-800-642-3111
Fax: 1-603-584-1681
www.boschhotwater.com
Data subject to change without notice | Printed in the USA | BTC 710002322 A | 08.2009
Bosch Thermotechnology Corp.