J.D. Hodges

Rheem Prestige Furnace – Error 45 (Low Pressure Switch Open) Fix Under $15

Our Rheem Prestige gas furnace decided to quit on us recently and locked out with Error 45 – Low Pressure Switch Open. No heat, just repeated startup attempts and the same code over and over. That flashing red 45 light was really bugging me 😅.

For context, this furnace (and original pressure switch) had been in service for about 8 years when the problem showed up.

I’m putting this post up in case somebody else is staring at a cold house, a flashing 45, and wondering where to start.

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What Error 45 Usually Means

On Rheem/Ruud high-efficiency furnaces, Error 45 (Low Pressure Switch Open) usually means the control board isn’t seeing the correct pressure signal when the inducer motor is running.

In normal operation, the sequence is roughly:

• Thermostat calls for heat
• Inducer fan starts
• Pressure switch closes when proper draft/pressure is detected
• Ignitor lights, gas valve opens, burners fire

If the control board doesn’t see the low-pressure switch close in time, it throws Error 45 and shuts the system down.

DISCLAIMER: I am a homeowner, not a licensed HVAC technician. After doing my research and before doing anything major I called my HVAC buddy just to make sure he thought it was a good plan. 👍 This guide is for informational purposes based on my experience. Always turn off power and gas before working on your furnace. If you aren’t comfortable, call a pro. 💯

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“Normal” Things to Check First

Before grabbing parts, it’s worth running through the standard checks HVAC techs (and service manuals) usually recommend:

1) Check the Filter

A clogged or overly restrictive filter can affect airflow and pressure.

• Confirm the filter is clean and properly sized

2) Inspect Intake & Exhaust Pipes

High-efficiency furnaces rely on clear PVC intake/exhaust runs.

• Look for: Leaves, snow drifts, bird nests, or even plastic bags.
• Check terminations: Go outside and make sure the PVC pipes coming out of your house aren’t blocked by shrubs or new siding.

3) Verify Condensate Drainage

Clogged traps or drain lines can interfere with pressure readings.

• Confirm the condensate trap isn’t full of debris
• Make sure the drain line flows freely
• Check for low spots where water could collect

4) Inspect Pressure Switch Tubing

The small rubber/vinyl tube between the inducer and pressure switch can cause problems if it’s:

• Cracked or brittle
• Full of water
• Kinked or loose

Remove it, check for obstructions, and reseat it securely.

If all of the above checks out and the furnace still throws Error 45 consistently—especially on an older unit—you may be dealing with a failed pressure switch, not a system issue.

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When the Actual Problem Is the Pressure Switch

After going through the usual suspects, the behavior was still:

• Inducer comes on
• Tries to start
• Falls back to Error 45 – Low Pressure Switch Open
• Repeats

At that point, it became pretty clear the low-pressure switch itself had failed.

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The Replacement Part (Low Pressure)

For our Rheem Prestige, the correct modern replacement low-pressure switch was:

PD425144
(Replaces Rheem part 42-102069-06)

Cost: Under $15

We had to call around a bit, but an HVAC supplier in Springfield, MO had one in stock. My wife was already in that town for other reasons, so she picked it up (normally a long drive for us).

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Installation – Easier Than Expected

Swapping the switch was very straightforward:

• Shut off power to the furnace
• Locate the low-pressure switch (small round/oval device with two wires and a rubber tube)
• Take a quick photo for reference
• Disconnect the two wires
• Remove the mounting screws
• Pull off the pressure tubing
• Install the new switch in the same position
• Reattach tubing and wires
• Restore power and call for heat

The furnace fired immediately and has been running perfectly ever since.

No more Error 45. No drama 🙌.

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Final Thoughts

If your Rheem furnace is throwing:

• Error 45 – Low Pressure Switch Open, and
• You’ve already checked the filter, intake/exhaust, condensate lines, and tubing,

there’s a very good chance the pressure switch has simply aged out, particularly after 7–10 years of service.

For us, a sub-$15 part and about 10 minutes of work brought the furnace back online and solved the issue completely.

If you’ve seen Error 45 (or 55) caused by something else—or have another solid troubleshooting tip—drop a comment. The next poor soul Googling this at midnight will thank you.

–J.D.

💡 PS: I’m keeping a couple spare parts on hand now, because they’re cheap and easy to swap:

• Low pressure switch: PD425144 (replaces 42-102069-06)
• High pressure switch: 42-102070-18 (verify for your exact model)
  
  ✅ Double-check your specific furnace model number on the interior data sticker before ordering, as part numbers can vary by BTU rating.

PSS something I was recommended after the fact: The “Suck” Test: A common diagnostic trick for pressure switches is to gently suck on the hose connected to the switch to hear if the diaphragm “clicks.” Of course the best way is a manometer but I didn’t have one on hand.