Unit Starts Clean But Runs Less Than 10 Seconds - Lennox G21Q3-40 Unit Information

CHECK FOR RECIRCULATION OF EXHAUST GASES TO
AIR
INTAKE AT THE OUTSIDE TERMINATIONS OF PVC PIPING.
ARE INTAKE AND EXHAUST PVC LINES SEPARATED
NO MORE THAN 3" AT OUTSIDE TERMINATION? DOES EXHAUST
TERMINATION EXTEND AT LEAST 8" PAST INTAKE TERMINATION?
NO
MODIFY OR COR ARE INTAKE AND EXHAUST PVC LINES TERMINATED
RECT INTO A WINDOW WELL, ALCOVE OR CORNER WHERE
SEPARATION TO A VARYING CONDITIONS CAUSE RECIRCULATION?
MAXIMUM OF 3".
IF OUTSIDE TEMPERATURE IS LOW ENOUGH,
OBSERVE EXHAUST OUTLET VAPOR WHEN
UNIT IS RUNNING. RECIRCULATION WILL
EASILY BE SEEN.
MODIFY OR CORRECT
TERMINATIONS TO
ELIMINATE RECIRCULATION.
CHECK SENSOR WIRE
REPLACE IF DETERIORATED
& RETEST UNIT
REMOVE & CHECK SPARK PLUG (USE LENNOX
PART #20H43 OR 3/4" SPARK PLUG SOCKET).
1. WAS PLUG TIGHT WHEN REMOVED?
2. ARE CRACKS PRESENT IN PORCELAIN?
3. REPLACE SENSOR IF
CRACKED?

UNIT STARTS CLEAN BUT RUNS LESS THAN 10 SECONDS

RESET UNIT IF LOCKED OUT, LISTEN FOR CHANGE IN SOUND
OF UNIT BEFORE IT STOPS.
UNIT LUGS" DOWN
BEFORE STOPPING.
RECIRCULATION
CHECKS
YES
NO YES
MODIFY OR CORRECT
TERMINATIONS TO
ELIMINATE RECIRCULATION.
NOTE: RECIRCULATION IS ALSO POSSIBLE
WHEN EXHAUST C02 CONTENT IS ABOVE
10%. THIS IS DIFFICULT TO MEASURE AS
AN INDICATION IN THIS CASE IF UNIT WILL
ONLY RUN FOR 10 SECONDS OR LESS.
IS FLAME SIGNAL PRESENT AND CORRECT?
CHECK WITH MICROAMP METER
NO YES
MONITOR MANIFOLD PRESSURE OR GAS VALVE COIL, PURGE BLOWER (120 VAC),
START HERE
UNIT STOPS WITHOUT ANY CHANGE
IN SOUND BEFORE STOPPING.
IS DIFFERENTIAL
CUTTING OUT UNIT?
NO
MAKE RECIRCULATION CHECKS.
IF RECIRCULATION IS NOT PRESENT, CHECK FOR ADEQUATE
GROUNDING OF UNIT AND PRIMARY CONTROL. IF OK, CHECK
PRIMARY CONTROL TIMING:
SPARK PLUG WIRE WITH SPARK TESTER AND FLAME SIGNAL. USE THE TIMING
CHARTS (FIGURE NO TAG ) TO DETERMINE IF THE CONTROL IS DEFECTIVE.
IF DEFECTIVE REPLACE
PRIMARY CONTROL.
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ARE LIMITS CUTTING OUT UNIT?
CHECK FOR OPEN LIMIT
5
IMMEDIATELY FOLLOWING UNIT
CUTOUT. SHUT OFF POWER
AND MAKE OHM METER CHECK.
NO YES
DETERMINE CAUSE OF LIMIT CUT OUT
AND CORRECT PROBLEM.
SWITCH
MAKE OHM METER CHECK ACROSS
TERMINALS IMMEDIATELY AFTER
UNIT SHUTS OFF. METER READS
4
( )OHMS WHEN SWITCH
CUTS UNIT OUT.
YES
CHECK FOR PARTIAL BLOCKAGE
OF ALL PVC PIPING
& CONDENSATE LINE.
IF NO BLOCKAGE EXISTS
REPLACE DIFFERENTIAL
PRESSURE SWITCH