Bosch A2 Error Code Troubleshooting

User Manual: Bosch A2 error code troubleshooting

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Model: 250SX, 250SXO, 635ES, 635ESO, 2400E, 2400EO

A2 Error Code Troubleshooting

Service Bulletin: TWH-G2-30

For Serial Numbers Through FD 689:

Introduction:

• An A2 error indicates a problem related to the white overheat

cutoff fuse that wraps around the heat exchanger.

• The fuse is a safety device that is designed to fail if the

chamber outside the heat exchanger experiences high

temperatures (approx 355 °F or 180 °C) due to an exhaust leak.

• An A2 error may indicate an exhaust leak from the combustion

chamber or may simply indicate a bad overheat cutoff fuse.

Overheat Cutoff Fuse

• The overheat cutoff fuse is a thick white wire (similar to a

fusible link) that wraps around the heat exchanger.

• When the fuse is intact, the heater senses continuity through

the fuse, indicating the fuse is okay.

• When the fuse is broken internally (or melted due to high

temperatures), there will be no continuity and an A2 error code

will be generated.

Tools needed:

• Ohmmeter/ multimeter

• Phillips head screwdriver

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. Conrm that the heater is installed according to the

specications in the manual.

a. The exhaust vent must be sealed single wall, AL29-4C

stainless steel not to exceed 26 feet.

b. The air intake pipe must not exceed 26 feet.

3. Inspect the heat exchanger for signs of exhaust leakage:

a. Perform a visual inspection of the heat exchanger, looking for

any signs of cracks or burn marks.

b. Visually inspect the gaskets on the top and bottom of the

heat exchanger to ensure the gaskets are properly seated.

c. Inspect the gaskets for any burned or melted sections.

d. Inspect the viewing window on the heat exchanger for any

cracks in the window or breaks around the window gasket.

4. Verify that the plug connector between the overheat cutoff fuse

(red wires) and the control unit is rmly connected (Fig. 1).

5. Disconnect the plug connector and check the resistance on the

white overheat fuse (red wires, see Fig. 1):

a. A good fuse should read approximately 1 Ω.

b. A failed fuse will read as an open line (no continuity).

6. If items 1 through 4 check out okay, but the fuse measures as an

open line, the A2 error may have been caused by a pinched or

failed fuse.

7. If the fuse has failed (open line), it should be replaced with a

retrot ue gas limiter (Part number 8 700 400 032). The ue gas

limiter looks very different than the overheat cutoff fuse but serves

the same purpose.

8. Refer to Bulletin TWH-G2-29 for installing the retrot ue gas limiter.

9. The overheat fuse and the ue gas limiter should not be confused

with the overheat sensor (ECO) on the upper right corner of the

heat exchanger. (The ECO serves an entirely different function and

prompts a different – E9 – error code.)

Overheat fuse

Red wires

Black wires

Figure 1

O

verheat

fuse

R

Re

d wir

es

B

lack wires

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| TWH-G2-30 | 250SX, 250SXO, 635ES, 635ESO, 2400E, 2400EO Service bulletin

Bosch Thermotechnology Corp.

Data subject to change without notice | Printed in the USA | BTC 711002310 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

For Serial Numbers FD 690 and Later:

Introduction:

• An A2 error indicates a problem detected by the ue gas sensor

located in the upper right corner of the heater.

• The ue gas limiter is a safety device that is designed to trip

if the chamber outside the heat exchanger experiences high

temperatures (approx 220 °F or 104 °C) due to an exhaust leak.

• An A2 error from the ue gas limiter indicates a possible

exhaust leak from the combustion chamber.

Flue Gas Limiter

• The ue gas limiter trips (creating an open circuit) when

temperatures in the upper chamber exceed 220 °F.

• The limiter will reset itself (closing the circuit) when it cools off,

but the error will still have to be reset manually using the reset

button on the control board.

Tools needed:

• Ohmmeter/multimeter

• Phillips head screwdriver

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. Conrm that the heater is installed according to the

specications in the manual.

a. The exhaust vent must be sealed single wall, AL29-4C

stainless steel not to exceed 26 feet.

b. The air intake pipe must not exceed 26 feet.

3. Inspect the heat exchanger for signs of exhaust leakage:

a. Perform a visual inspection of the heat exchanger, looking

for any signs of cracks or burn marks.

b. Visually inspect the gaskets on the top and bottom of the

heat exchanger to ensure the gaskets are properly seated.

c. Inspect the gaskets for any burned or melted sections.

d. Inspect the viewing window on the heat exchanger for any

cracks in the window or breaks around the window gasket.

4. Verify that the plug connector between the ue gas limiter

(white wires) and the control unit (black wires) is rmly

connected (see Fig. 2).

5. The ue gas limiter should not be confused with the overheat

sensor (ECO) on the upper right corner of the heat exchanger.

(The ECO serves an entirely different function and prompts a

different – E9 – error code.)

Black wires

White wires

Red silicone

grommet

B

lack

wir

es

W

h

ite wires

R

ed

s

i

l

i

co

n

e

g

romme

t

Figur

e 2