have a goodman model gmpn080-4 I lost blower limit pressure switch previously and seemed to be the same thing, ordered part and still does same thing. thought it may have had a vacuum leak so I sealed the edges, as long as it is not connected (securely) to the exhaust pipe it runs fine. As soon as I tighten down it does the thee blink sequence that says a bad switch or blockage. Has anyone seen this happen? The exhaust pipe is clear?

Check both the air intake as well as the exhaust. Also make sure air filter is clean and condensate is draining and all hoses are clear of debris.

Make sure the fittings that the pressure switch tubing are attached to are open and not clogged up. Good luck, Mike

thanks for the responses, yes I have triple checked all my connections and have good air circulation. One thing I thought is the exhaust blower motor may not have enough power with the back pressure of the exhaust, and that's why it kicks the code?

This is a vent blockage, plain and simple. If it works fine with the exhaust removed, but fails when you connect it, there is a venting problem. You will need to make *sure* the vent is clear, and check the existing venting against the installation instructions for that model furnace. Are there long horizontal runs with "bellies" in the pipe? Is there a reducer in a horizontal run? Is there a screen on the outdoor side preventing squirrels and mice from nesting in the pipe?

You may need to get a tech to look at this-and I don't recommend you call the people that installed it. If they put it in wrong, they aren't going to be that quick to tell you.