Is it normal to condemn a furnace over 100 ppm flue CO without seeing a crack?

100 ppm of CO in the flue pipe is absolutely not an automatic death sentence. The standard industry cutoff and Goodman's own threshold for tagging a furnace out of service is 400 ppm air-free. While 100 ppm is on the higher side of what we like to see on a clean burn, plenty of Goodman units run near 80 to 100 ppm right out of the box. Condemning an eight-year-old unit purely on an assumed "hairline fracture" without finding a single crack, hole, or flame rollout is jumping the gun. Carbon monoxide in the exhaust only tells you one fundamental thing: the fuel-to-air ratio is off. If that vent pipe is legitimately choked with too many fittings, that restriction alone starves the burner of proper airflow and creates incomplete combustion. High CO is also frequently caused by simple overfiring from high manifold gas pressure or dirty burner faces. Did the tech acutally hook up a manometer to test gas pressure, or run a **full combustion analysis** showing O2, excess air, and flue temp? Sticking a sniffer in the PVC, seeing double digits, and guessing there's invisible damage is a classic sales move.

definately pay for a second opinion from a service technician who knows how to read combustion numbers, not a tech on commission. Have them clock the gas meter, verify the manifold pressure against the rating plate, and actually calculate the equivalent pipe length from Goodman's manual. A couple of 45-degree elbows ussually only add about two and a half equivalent feet each, so your venting might not even be out of spec.

Even if there were a cracked cell—which seems unlikely given the inspection—that heat exchanger still carries a 20-year parts warranty. Dropping five grand to trash an eight-year-old furnace over a reading that passes manufacturer thresholds makes zero sense until you've checked the basics first.

Totally agree with everthing above. One simple check that tech completly skipped: watching the flame pattern when the indoor blower motor kicks on. If there were an actual fracture between the heat exchanger and the living airstream, the house fan pressurizing the cabinet woud visibly disturb the flames or push them back toward the burner face. If the flame stays rock steady before and after the blower starts, his 'invisible crack' story falls apart real fast.

Also make sure the second tech checks the condensate trap. Sludge backing up water into the secondary collector box will restrict draft and drive up CO just like a pinched pipe.

Did he even bother inspecting the internal condensate trap and the collector box? On a 92% condensing platform like the GMSS92, inadequate drainage—whether from sludge accumulating in the molded trap or insufficient slope on the horizontal PVC run—causes acidic water to back up directly into the finned secondary heat exchanger, which restricts flue passage volume and routinely elevates carbon monoxide without indicating any structural failure of the cells. I would pull and flush that trap assembly thoroughly and ensure the exhaust is pitched at least a quarter-inch per foot back toward the furnace before entertaining an entire equipment swap.

Have you ruled out an issue on the intake side rather than the exhaust? If your Goodman is piped as a two-pipe direct vent, a partially blocked intake screen outside—leaves, spiderwebs, or frost—will starve the burner of fresh air and spike CO just as easily as tight bends.

Also, did you catch where he zeroed his tool? If he calibrated the analyzer inside your basement or right next to a water heater instead of outside in fresh air, that 100 ppm baseline coud be skewed from the jump. I'd defintely want those simple variables checked before dropping five grand on a swap.