INTRO
0:00-0:30
ON-SCREEN: D24-misdiagnosis-triangle.svg, title visible
Three different faults. One identical symptom. A system low on charge, a TXV that will not feed, and a choked filter all show you the exact same thing on a low-side gauge: low suction pressure. Techs who read one gauge call all three "low on refrigerant," and two out of those three times they are wrong, and the refrigerant they add makes the system worse. In the next four minutes you will learn the two questions that split these three faults apart every single time, plus the signatures for overcharge, air in the system, and restrictions. This is the module that keeps you off the gas-and-go list.
MAIN
[0:30-1:30] The triangle and the two questions
ON-SCREEN: D24-misdiagnosis-triangle.svg, camera pushes from center "LOW SUCTION" label to each corner as named
Here is the misdiagnosis triangle. Center: low suction pressure, the shared symptom. Corner one: true low charge, the refrigerant actually left through a leak. Corner two: TXV underfeed, the valve is pinched and refrigerant cannot reach the coil. Corner three: low airflow, the coil cannot find enough warm air to boil the refrigerant it is getting.
Two questions break the triangle. Question one: superheat, high or low? Recall from F6: superheat measures how starved or full the evaporator is. Low charge and a pinched TXV both STARVE the coil, so superheat runs high. Low airflow floods the coil with liquid that cannot boil, so superheat runs LOW. Low superheat sends you to the air side immediately.
Question two, only if superheat was high: subcooling, low or stacked? Subcooling is the condenser dipstick. Low charge means less liquid everywhere, dipstick reads near empty. A pinched TXV dams refrigerant up into the condenser, dipstick reads normal or high. Missing versus trapped. That is the whole discrimination.
[1:30-2:30] The seven readings and the healthy baseline
ON-SCREEN: D24-seven-readings-dashboard.svg, each tile highlights as named
You answer those questions with the seven-readings routine: suction pressure, head pressure, superheat, subcooling, line temperatures, indoor temperature split, compressor amps, all judged against outdoor ambient.
Here is the healthy picture on a 3-ton R-410A system, 100 degree Phoenix day, 78 degree return air. Suction 130 psig, which the PT chart calls a 45 degree coil. Head 390 psig, a 115 degree condensing temperature, 15 over ambient, right in the healthy 15-to-30-over band. Superheat 10. Subcooling 10. Split 20 degrees, inside the 18 to 22 window. Amps comfortably under nameplate. Burn this shape into your memory, because every fault is a distortion of it.
[2:30-3:30] The three signatures side by side
ON-SCREEN: D24-fault-signature-table.svg, rows highlight as named
Same day, three faults. Low charge: suction 102, superheat 26, subcooling 3, head low, split weak, amps low. Everything says "not enough refrigerant anywhere."
TXV underfeed: suction 102, superheat 24, nearly identical so far, but subcooling 14. The refrigerant exists. It is dammed up behind the valve. Add charge here and subcooling climbs while suction barely moves.
Low airflow: suction 108, but superheat 4. Four degrees. A starved coil cannot do that; only a flooded one can. Subcooling 10, dead normal. The split goes weird: high at first, because the trickle of air leaves the coil extra cold, then the coil ices and everything collapses. Add charge here and you feed the ice and chase superheat toward floodback.
[3:30-4:15] Overcharge, air, restrictions, and the NIST receipts
ON-SCREEN: D24-restriction-and-noncondensables.svg, then D24-nist-fault-sensitivity.svg
Three more signatures round out the circuit. Overcharge: high subcooling, high head, high amps, and on a TXV the superheat sits there looking innocent. Non-condensables, meaning air left in by a skipped vacuum: high head that sits ABOVE what the PT chart predicts, condensing 40 or 50 over ambient, and the killer check is standing pressure: a system soaked overnight at 100 degrees must stand at about 317 psig, and air pushes it higher. Restrictions: high superheat, high subcooling, and the address written in temperature, a cold or frosting filter drier with a measurable drop across it.
And the lab agrees. NIST measured which readings move first for each fault: subcooling fell almost 88 percent at 30 percent undercharge, the loudest alarm in the whole study, while performance barely budged until the faults got deep. The numbers move a full season before the comfort complaint. Take all of them.
OUTRO
4:15-4:30
ON-SCREEN: D24-fault-signature-table.svg with text overlay: SEVEN READINGS. TWO QUESTIONS. NAME IT BEFORE YOU OPEN IT.
Seven readings, two questions, and the fault has a name before a cylinder ever leaves the truck. In the demo video, Darrel stages these exact faults on a live unit and calls them blind from the numbers. Watch how fast a full picture turns into a verdict.