Fault Tree: A/C Not Cold

Quick triage & tools

Scan first: read codes (all modules if you can) + live data that matches the symptom.
Power basics: battery voltage (resting + while cranking) and quick fuse checks before deep dives.
Have ready: basic scan tool, multimeter, and a way to confirm the symptom (road test / idle test).

Tip: if a step says “check wiring”, use Wiring diagrams basics → and Voltage drop testing →

Safety note

Refrigerant can cause frostbite and is regulated in many regions. If you’re not qualified, focus on the electrical/command checks and basic airflow/blend-door checks first, then use a licensed A/C tech for recovery/charging.

Decision flow

Confirm airflow & cabin controls
- Blower works on multiple speeds? (If not, fix blower/resistor first.)
- Temperature knob/setting changes anything? If stuck hot, suspect blend door actuator.
Is A/C requested and allowed?
- A/C button on + light on? Defrost mode often forces A/C request.
- Ambient temp very low? Many cars inhibit A/C below ~3–5°C.
- Any engine overheat? ECU may disable A/C if coolant temp is high.
Compressor command / clutch (if fitted)
- No clutch click / no compressor response → check A/C fuse, relay, and request signal.
- Variable displacement compressors may have no clutch; look for ECU command to the control valve.
Condenser fan operation
- Fan not running with A/C request? Many systems won’t cool properly; fix fan control first.
Pressure sensor plausibility (scan tool)
- 0 bar / implausible → wiring/sensor fault or system empty (leak likely).
- Normal static pressure but no cooling → compressor not pumping or airflow/blend issue.
If you have manifold gauges (trained users)
- Low side very low, high side low → low charge / restriction / compressor weak.
- Low side high, high side low → compressor not pumping / control valve stuck.
- High side very high → condenser airflow problem, fan issue, overcharge, restriction.
Cold evaporator but warm cabin?
- If pipes at the bulkhead are cold but cabin is warm → blend door / heater valve / air routing issue.
- If cabin starts cold then warms → evaporator icing, low airflow, or sensor/control issues.

Back to Fault Trees.

Print / save checklist

Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.

Freeze-frame captured / conditions noted
Battery voltage checked (resting + cranking)
Basic visual checks (hoses, connectors, grounds, fuses)
One test at a time (don’t change multiple variables)
Confirm fix by reproducing the original condition

Print / Save as PDF Back to symptom selector

What to do next

Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.

Find another symptom flow Jump to the symptom selector to locate the closest decision tree. Workshop Guides Deep-dive how-to tests: voltage drop, wiring diagrams, smoke testing, fuel pressure and more. Diagnostic Codes Look up DTC meanings, common causes, and related checks. AI Tools Use AI assistance to summarise symptoms, plan tests, or sanity-check a diagnosis.