Looks to me like the PVC pipe is dripping condensation back into this unit. Do you guys agree? You can see some significant staining at interior of the unit on bottom left.
I see the evidence of condensate dripping. It is ofter the case on high efficiency furnaces on the exhaust side. But, not so much on the intake side.
Was there a missing elbow at the intake exterior termination?
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Yep, it is right below that intake… 
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The connection at the exhaust fan appear loose…
The pan below the Evaporator Coil might be clogged.
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I could not locate exhaust vent or air intake on exterior. This was actually the 1st floor of a large condo complex. The PVC vents appeared to run up 2 stories to the roof. Could not get a visual of the roof however.
Thanks Brian. I double checked those connections and they seemed pretty secure.
Cool, smart to check those out. If you are going to attempt to diagnose this issue I would recommend you tread lightly.
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[quote="Erik Armendariz, post:7, topic:239131, full:true, username:earmendariz”]
[quote="lkage, post:3, topic:239131”]
Was there a missing elbow at the intake exterior termination ?
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I could not locate exhaust vent or air intake on exterior. This was actually the 1st floor of a large condo complex. The PVC vents appeared to run up 2 stories to the roof. Could not get a visual of the roof however
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Then, like Brian said:
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Definitely good advice. As you know, as inspectors we often find ourselves in those positions when talking with clients - especially when they are under the impression we’re supposed to know the defect and the diagnose.
That’s where managing their expectations comes in.
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There was a townhouse development where every single Goodman unit we saw at 11 month inspections had this corrosion. It was condensation dripping down the intake pipe during the summer time. Several furnaces had issues because the water damaged some electrical component directly under the intake opening. The fixes I heard about all involved adding insulation to the pvc pipe and the utility chase that were in, but I don’t know how that has worked out long term.
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It’s a trap, lol. Larry has said it well, manage expectations.
I say things like “it could be one or more issues here, we need to get a good HVAC tech in here to fully diagnose”
It’s ok to remind them we are generalists.
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this was a Goodman townhouse/condo.
Yes, that is a problem. Was there a wasp nest inside the combustion air inlet pipe?
When I had my American Standard, high efficiency furnace installed. It was recommended to add a pigtail on the air intake for catching condensation. Evidently carrier has had a problem with this and even designed a part for their units.
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Good info James. 
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Erik - The makeup air on this unit is not connected directly to the induced draft fan.
The condensation is coming from air back drafting up the vent in the off cycle. Damp air inside the house is condensing as it reaches the attic and outdoor temperatures, where it condenses. This can be from natural convection or positive indoor air pressure.
Outdoor air in the heating season has much less moisture content than the indoor air. Not to be confused with Relative Humidity readings. As the OD Air enters the pipe, it becomes warmer on its way to the furnace and will not condense because the dew point becomes much lower.
The furnace is not airtight at that access panel to prevent this. There are holes everywhere into this location.
You may want to check for potential positive pressures when you find this. Also, the termination of the
Pipe above the roof (rain water).
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That digram is really helpful. Thank you for that. I may even include this in future reports to help the clients. Thanks James