Precedent with Electromechanical controls, Small (3-5T) and Large (15-25T) Foundation units utilize a single gas heat ignition board for the control of the supply fan and gas heat.
Through the years, the boards have changed in their look, but operate the same.
Older units will use CNT04717
Newer units will use CNT05135
Differences in these two boards will be called out as necessary through out the document,
Is my unit in Heat Mode?
- For thermostat control units, 24 vac on W1 and/or W2 would be heat mode
- A unit with any cooling call with a heating call will not heat until the cooling call is removed
Does the IGN Module have power?
The IGN Module has a status LED that gives the status of the module. The placement of the Status LED varies based on the age of the ignition module as well as the design.
CNT04717 Ignition Boards
IGN LED1 will be solid green with no heat call and in a slow pulse (3/4 sec on, 1/4 sec off) for a heating call. LED1 will also blink codes (1/4 sec on, 1/4 sec off with a 2 sec pause at the end of the code) for diagnostics. See the link below for details on LED1 blink codes
Precedent Electromechanical Ignition Board Info and Troubleshooting Tips
CNT05135 Ignition Boards
On the CNT05135, there are two LEDs, Status and Fault.
The operation of the Green Status LED is as follows
- Slow Flash: Normal operation with a cool or fan only call
- Fast Flash: Normal operation with a call for heat only
The operation of the Red Fault LED is as follows:
- Solid Red: Internal control board failure, replace board
- Solid Red w/ Solid Green Status: Continuous Reset, caused by internal board failure, replace board
- Flashing code: See below
If LED1 of the Status LED is out, this indicates that there is no power to the ignition module.
On Foundation units, power comes from the backside of LTB1-R (Thermostat Terminal block) to IGN 24VAC (R) and the ground/neutral is connected to the B (TR)/B (GND)/COMMON terminal:
Note: Wire numbers will vary based on cabinet size and unit age, see unit wiring diagram for more details.
Two Flash Diagnostic on LED1 or Fault LED (¼ sec on, ¼ sec off, 2 times w/ 2 sec pause)
Failure to sense flame or Lockout
A 2 flash diagnostic indicates that either no flame was sensed on ignition or that the flame dropped out while running. Troubleshooting for this issue would be the same for any gas heat unit that uses a flame sensing rod or element for proving.
- Verify that the gas is on to the unit and the inlet/line pressure to the gas valve is between 4.5" - 14"
- Verify that gas valve opened and that manifold pressures are set correctly (see unit IOM for pressures)
- Verify that ignitor is working and that there is gas flow
- Verify the flame sensor operation
Precedent flame sensor troubleshooting
Flame Sensor Testing on Staged Gas Heat
Ignition Board CNT05135 includes a Flame Check point on the board that allows the use of a multimeter to measure the flame signal without disconnecting the flame sensor.
Three Flash Diagnostic on LED1 or Fault LED (¼ sec on, ¼ sec off, 3 times w/ 2 sec pause)
Pressure Switch failed to close or stay closed
A 3 flash diagnostic indicates that the the pressure switch did not close in the correct amount of time after the combustion fan was energized, opened while the unit was heating or is closed when the combustion fan is commanded off.
Combustion Fan not running
The combustion fan Foundation units is a 2 stage 208-230/1ph capacitor start motor. On 460/575vac units, power for the combustion fan motor is supplied by a step down transformer.
The ignition module takes one leg of power in on L2 and switches the L2 leg between Lo or Hi/Single to operate the fan at high or low speed. The other leg of power is applied directly to the motor (typically the black wire) and is hot at all times.
CNT04717
CNT05135
Combustion Fan Runs, but I'm getting a still getting a 3 blink code on the ignition module
If the combustion fan is coming on, but still not making the pressure switch, verify the following:
- pressure switch tube and pickup port on heat exchanger/combustion fan housing are clear of debris and moisture
- Verify the Pressure Switch has 24vac from one side of the Flame Rollout Switch from 24 VAC OUT at all times on.
- The 24 vac path is shown below:
- The 24vac for all the heat safeties are daisy chained to one side of each switch with the output of that switch going to it's respective input on the ignition module.
- Using a manometer, tee into the pressure switch line to validate the pressure the switch is seeing. The switch trips at 0.15" wc +/- 0.07" wc
- The switch opens ~ 0" wc
- If pressures are not high enough to actuate the switch, verify that the exhaust flue is open and not obstructed and that the burner openings are also not obstructed
No call for heat, but I'm getting a still getting a 3 blink code on the ignition module or the 3 flash code starts at the same time as the heat call
This fault can also be caused by a pressure switch that is closed when the combustion fan is not commanded on or jumped out.
Verify that the switch is wired for Normally Open operation, closes on pressure and that there are not jumpers present across the pressure switch terminals/wires. If testing shows the switch closed with the wires removed, replace the pressure switch.
Four Flash Diagnostic on LED1 or Fault LED (¼ sec on, ¼ sec off, 4 times w/ 2 sec pause)
High Limit switch protection device open
A 4 flash diagnostic indicates that one or both of the Thermal Cutout (TCO) switches are open during a call for heat. TCO lockouts are auto reset and will do not require a manual reset. When a TCO trips, it will keep the supply fan on, run the combustion fan at high speed.
TCO1 receives power from the common power side of the pressure switch. They are wired in series where TCO1 feeds 24vac to TCO2. Trouble shooting should always start by verifying that 24 vac is present on both sides of TCO1 then down to TCO2.
The 24vac path is shown below:
TCO trips are typically caused by a low airflow situation or damaged TCO wiring/switch/mounting. TCO locations vary based on the unit size, but there are always 2. TCO1 will be located in the burner section and TCO2 will be installed in the side of the indoor fan housing.
Airflow adjustments are made at the motor sheave typically.
TCO2 on Precedent with Electromechanical controls with a plenum fan is mounted on the frame of the plenum fan and requires removing the roof of the unit for access.
Where is TCO2 located on a Precedent 6 - 10 ton Legacy Precedent with the BC Plenum Motor?
Supply fan speed changes on these units are made through the ECM engine (blue board).
How do I change the motor speed on my Electromechanical Precedent with the ECM (MOD02127)
Five Flash Diagnostic on LED1 or Fault LED (¼ sec on, ¼ sec off, 5 times w/ 2 sec pause)
Flame sensed and gas valve not energized, or flame sensed and no call for heat.
A 5 flash diagnostic indicates that the ignition module is sensing a flame without a call for heat. This could be a failing flame sensor or a gas valve that is not closing. When the ignition board is in a five flash, the combustion fan will be on at high speed.
Troubleshooting would include checking the signal from the flame sensor as shown in the troubleshooting for a 2 flash diagnostic shown above.
If the gas valve is not closing, immediately isolate/close the gas valve from the unit
If voltage is present remove the low voltage connection from the valve and check voltages again from the board, stray voltage from the board with no call requires ignition board replacement and locking the gas valve out until the board is replaced.
Verify wiring between the ignition board and gas valve.
If voltage is not present and the valve is staying open, lock out the gas until the valve is replaced.
Six Flash Diagnostic on LED1 or Fault LED (¼ sec on, ¼ sec off, 6 times w/ 2 sec pause)
Flame Rollout Switch open
A 6 flash diagnostic indicates that the temperature at the top of the heat exchanger has reached the trip limit and or there is an actual flame on the top of the burner (rollout).
The Flame Rollout (FR) receives power from IGN 24 VAC OUT. Trouble shooting should always start by verifying that 24 vac is present on both sides of FR.
The 24vac path is shown below:
Troubleshooting flame rollout includes:
- Check heat exchanger for cracks
- Check for dirty/stopped up heat exchanger (typically caused by improperly set gas pressure, set too high)
- Check gas pressures, see IOM for settings
- Check combustion fan operation and verify that it is running in both speeds and is staying on at all times. If the combustion fan is shutting off and the heat is not shutting off in 5-9 seconds after the fan loses power, a functional check of the pressure switch is necessary
A tripped flame rollout requires a manual reset at the flame rollout switch.
If the switch, wiring, burner or any other part of the burner is compromised from a flame rollout, close and lockout the gas valve until repairs can be completed.
Seven Flash Diagnostic on LED1 or Fault LED (¼ sec on, ¼ sec off, 7 times w/ 2 sec pause)
Heat Call Swapped (W2 before W1) - CNT04717 Only
A 7 flash diagnostic indicates that IGN has detected a W2 call without a W1 call, verify thermostat wiring.
CNT05135 External Gas Valve Circuit Error
A 7 flash diagnostic indicates that the IGN has detected a miswired or shorted gas valve. Check gas valve wiring.
Eight Flash Diagnostic on LED1 or Fault LED (¼ sec on, ¼ sec off, 8 times w/ 2 sec pause)
Low Flame Sense - CNT05135 only
A 7 flash diagnostic indicates that the IGN has detected a weak flame while the heat is on. This is represented by a flame sensor reading of 0.7 vdc on the Flame Check terminals on the IGN.
Troubleshooting should start with visual flame inspection looking for lazy or weak flame which could be caused by improper gas pressure settings or poor airflow through the heat exchanger.
- Verify gas pressure
- Verify that flue and associated flue piping are clear
- Verify that flue screen is clear
- Verify that combustion fan motor is wired correctly
- Clean and inspect flame sensor