A.O. Smith TC-099 User Manual

AOS WPC - Tech Training

57 of 72

Ashland City, TN © 2007

Servicing should only be performed by a Qualified Service Agent

Flame or ignition failure.
Error message displayed after 1 or 3 losses of flame 
or after 1 or 3 failed trials for ignition. EMC 5000 
control system will lock-out after 1 or 3 trials 
depending on the SW1 #2 dip switch setting on the 
MCB circuit board. (see page 30)

When performing any troubleshooting steps outlined 
in this service manual always consider the wiring 
and connectors between components. Perform a 
close visual inspection of all wiring and connectors 
to a given component before replacement. 

Ensure wires were stripped before being crimped in 
a wire connector, ensure wires are crimped tightly in 
their connectors, ensure connection pins in sockets 
and plugs are not damaged or worn, ensure plugs 
and sockets are mating properly and providing good 
contact.

Failure to perform this critical step or failing to 
perform this step thoroughly often results in 
needless down time, unnecessary parts 
replacement, and customer dissatisfaction.

•

Ensure the supply gas shut off valve to the 
boiler and the gas train shut off valve (see 
image on page 23) are both open.

•

Perform the Start Up Procedure pages 17-19.

•

Perform the Poor Combustion/Ignition Problems 
procedures on pages 20-23.

•

View the burner through the view port near the 
igniter on the top of the combustion chamber 
during the ignition trial period. If the burner is 
igniting but the flame goes out in a few seconds 
ensure the flame sensor wiring is connected 
properly and in good condition. 

Note: There are two flame sensors on VF boil-
ers. A single wire is connected to the J16 con-
nect on the MCB (page 29). This wire is 
bifurcated (divided) into two wires and each end 
then connects to one of the two flame sensors. 
Repair/replace any worn or damaged wiring.

•

If the flame sensor wiring is in good condition - 
perform a flame sensing test. Using a DC micro 
amp meter; place the meter test probes in 
series with the flame sensor wire. Disconnect 
the wire at the J16 connect on the MCB (page 
29) insert the tip of one test probe into the wire 
connector securely - touch and hold the other 
test probe to the J16 connect. 

Measure the flame sensing current during igni-
tion. If the flame sensing current is less than 2.5 
µA - remove, inspect, and clean both flame sen-
sors. 5.0 µA flame sensing current is typical.

If either flame sensor shows signs of damage or 
the ceramic insulator is cracked - replace the 
flame sensor (s). If the flame sensors are in 
good condition clean both sensors with fine 
steel wool and reinstall both sensors. Check 
flame sensing current again to see if the current 
is higher and burner flame is established.

If the burner flame is still going out after a few 
seconds and all the above checks and proce-
dures have been performed - call the A. O. 
Smith technical information center for further 
assistance at 800 527-1953.