Pentair MasterTemp Shutting Off Mid-Cycle? Check the 42002-0024S Stack Flue Sensor
A heater that lights, runs for a few minutes, then shuts itself down is one of the more frustrating calls we get. The burner sounds fine. The water is moving. Nothing looks broken. On Pentair MasterTemp and Sta-Rite Max-E-Therm heaters, that pattern often points at one small part: the 42002-0024S stack flue sensor.
It is a cheap part that can take a whole heater offline. Here is what it does, how to check it, and when replacing it actually solves the problem.
What does the 42002-0024S stack flue sensor do?
The 42002-0024S is a temperature sensor that sits in the heater's exhaust stack and watches how hot the flue gases are. If exhaust temperature climbs past what the control board expects, the board shuts the burner down.
It is a safety part, not a performance part. The heater will not run without a working one, and it is designed that way on purpose. High flue temperature usually means heat is going up the stack instead of into your water, which points at a dirty heat exchanger or bad water flow.
Why is my MasterTemp shutting off after a few minutes?
Nine times out of ten it is one of three things: low water flow, a sooted or scaled heat exchanger, or a stack flue sensor that has drifted out of spec. The first two are real problems the sensor is correctly reporting. The third is the sensor lying to the board.
Before you buy a 42002-0024S, rule out the easy stuff:
- Dirty filter or a clogged pump basket cutting flow to the heater
- Valves partly closed, or a pump running at too low a speed on a variable speed setup
- Scale buildup inside the heat exchanger, common on hard water and salt pools
- Soot on the burner tray or a partly blocked exhaust path
- Rodent nests or debris in the stack, which happens more than people expect
If flow is good, the filter is clean, and the heat exchanger looks reasonable, then the sensor itself moves to the top of the list.
Signs the stack flue sensor is bad
A failed 42002-0024S usually shows up as a heater that runs briefly and then trips, over and over, with no obvious cause. The control panel on a MasterTemp has an indicator for the stack flue sensor, and it will light up when the board sees a problem there.
Other things we hear from customers:
- The heater trips faster and faster as the season goes on
- It runs fine on cool mornings but quits in the afternoon heat
- The wire leads or connector at the sensor look corroded, green, or brittle
- The heater will not fire at all and the stack flue indicator is on right from the start
Corrosion at the connector is worth a close look. The stack is a hot, damp place, and a bad connection reads the same as a bad sensor to the control board.
How to test a 42002-0024S before you replace it
Kill power at the breaker first. Not the heater switch, the breaker. Then pull the top panel and find the sensor threaded into the stack with two wires running back to the board.
- Unplug the sensor leads and inspect the terminals for corrosion or heat damage
- Set a multimeter to resistance and read across the two sensor leads with the heater cold
- An open circuit (no reading at all) or a dead short both mean the sensor is finished
- Compare your cold reading against the spec in your heater manual, since it varies by model year
- Check the harness back to the board for chafed or melted insulation while you are in there
If the sensor reads open, replace it. If it reads plausible but the heater still trips, the problem is upstream and a new 42002-0024S will not fix it.
Replacing the sensor
This is a straightforward swap for most people. Power off, unplug the old leads, back the sensor out of the stack fitting, thread the new one in, and reconnect. Do not overtighten it. Snug is enough, and cranking on it can crack the fitting.
Give the new sensor a clean, dry connection. If the old connector was corroded, clean the mating side before you plug in the replacement or you will be back here in a month.
Replacement cost and whether it is worth it
The 42002-0024S runs well under a hundred dollars, which makes it one of the cheapest fixes on a gas pool heater. Compare that to a new MasterTemp and the math is not close. Unless your heater is old enough that the heat exchanger is also going, replacing the sensor is the obvious first move.
Where we would push back is if you have already replaced this sensor once in the same season. Repeat failures mean something is running hot, and that is worth having a tech look at before you throw another part at it.
Other parts worth checking at the same time
While the panel is off, look at the high limit switches, the water pressure switch, and the ignition control module. These all sit in the same shutdown chain and can produce similar behavior. Cleaning the burner tray and checking the exhaust path costs nothing and often reveals why the flue got hot in the first place.
If your heater is tripping and you have narrowed it down to the stack flue sensor, the 42002-0024S below is the genuine Pentair part for MasterTemp and Max-E-Therm heaters.
Pentair | 42002-0024S | Stack Flue Sensor | Sta-Rite MasterTemp & Max-E-Therm Replacement
$62.99
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