AC not cooling: OBD-II diagnostics, refrigerant pressure, compressor and sensors

“AC not cooling” — driver goes to a shop, mechanic says “top up refrigerant”, and a week later it’s back. Actually there are 5-7 causes, and OBD-II sees most. Below: how to diagnose HVAC via a diagnostic adapter.

What makes up the AC system

• Compressor (with electric clutch) — squeezes gas to 220-290 psi
• Condenser — “radiator” in front of the main, cools gas to liquid
• Receiver-drier — filters moisture
• TXV / EXV — expansion valve
• Evaporator — under the dash, gas evaporates → cools air
• Sensors: high / low side pressure, evaporator temperature, air temperature
• HVAC module — controls everything via CAN

OBD-II diagnostics — which PIDs via Mode 22

Standard OBD-II shows almost nothing. Via Mode 22 UDS (manufacturer PIDs) on VAG/BMW/Mercedes/Toyota:

• AC high side pressure (psi) — with AC running: 115-220 psi (spec); >260 → overcharged or blocked condenser
• AC low side pressure (psi) — 29-58 psi (spec); below 22 → low refrigerant; above 72 → compressor can’t hold
• Evaporator temperature — 37-46°F (spec). Below 32°F → freezes, cycles off
• AC compressor duty cycle — 40-90% in hot weather (spec)
• Refrigerant weight (calculated) — on some cars the ECU estimates how much refrigerant remains (grams)
• AC clutch engagement request — is the ECU asking clutch to engage
• AC clutch actual state — did the clutch actually engage (for clutch diagnosis)

Key DTCs

Code	Meaning
B1234	AC high pressure switch fault
B1235	AC low pressure switch fault
B10DA	AC clutch relay fault
B12AB	Evaporator temperature sensor
P0645	AC clutch relay control circuit
P0532	AC refrigerant pressure sensor low
P0533	AC refrigerant pressure sensor high

Classic patterns

Not cooling — low pressure

• OBD: High side 58-87 psi (spec 145+), Low side 15-22 psi (spec 29-58), evap temp 60°F
• Cause: not enough refrigerant (leak or natural loss over 3-5 years)
• Fix: find the leak (UV dye + UV lamp, $20), then recharge ($50-100). DON’T RECHARGE WITHOUT FINDING THE LEAK — in half a year it’s gone again, plus diagnosis gets harder

Not cooling — compressor not working

• OBD: AC clutch engagement request = ON, AC clutch actual = OFF; or both OFF
• Cause: compressor clutch, relay, or the compressor itself
• Fix: AC relay ($10-30), clutch ($80-200), new compressor ($400-1200 + $200-400 labour)

Not cooling — overcharged

• OBD: High side 290+ psi, Low side 72+ psi, evap temp 50°F
• Cause: recharged “by eye” without a scale. Too much refrigerant = compressor can’t hold
• Fix: bleed excess, recharge properly (~$50)

Evaporator freezes (cools then doesn’t)

• OBD: Evap temp drops to 28°F, cycles off, rises to 68°F, cycles on
• Cause: clogged cabin filter or faulty evap sensor
• Fix: replace cabin filter ($10-30), or evap sensor ($30-80)

Always blows cold, no temp control

• Cause: HVAC module or blend-door actuator
• Fix: actuator $50-150; HVAC module $200-500

Prevention

• Every 3 years — professional service with vacuum and recharge (~$100)
• Every 1-2 years — cabin filter change ($10-30)
• In winter — run AC for 5-10 min every 2 weeks (so oil doesn’t settle)
• Don’t “top up from a can” without vacuuming — you’ll flood moisture, receiver-drier is toast

Our AI (DTC Analyzer) reads all 6 AC PIDs and returns a ranked hypothesis in 5 sec:

• “Refrigerant leak 68%”
• “Compressor can’t hold 22%”
• “Cabin filter clogged 10%”

Try it on the free tier — pair an ELM327 adapter. Standard B-codes — on every car. AC pressure PIDs via Mode 22 — on paid tier.