Symptom · Petrol engines with port or direct injection
Cold running uses different fuelling, so a fault that only shows cold is usually about mixture or a sensor the engine trusts before it has real data.
Coolant temperature sensor reading wrong
The engine fuels for the temperature it is told, not the one it has.
Vacuum leak that closes up when warm
A hard hose or a gasket seals as it expands, so the leak disappears with heat.
Weak or dirty injector
A poor spray pattern matters most when the mixture is already rich and cold.
Carbon on direct injection valves
Deposits soak fuel on a cold start and give it back once hot.
One low cylinder
Low compression shows up cold, when there is least heat to help it fire.
Nothing on that list is the answer yet. A measurement decides which ones survive.
Test 1
Read coolant temperature on the scan tool at a genuinely cold start and compare it with ambient.
A sensor stuck at 40 °C or -40 °C explains cold fuelling on its own.
Test 2
Watch short and long term fuel trim at idle from cold, then again once warm.
Trim that corrects as the engine warms is the signature of a leak that seals with heat.
Test 3
Smoke test the intake cold, before the engine has run that morning.
Testing cold is the point: a warm test can seal the very leak you are hunting.
Test 4
Run a relative compression test on a cold engine.
One low cylinder ends the fuelling hunt and changes the conversation with the customer.
Fuel trim from a cold start tells you more in two minutes than an hour of parts swapping.
Open a Case with the vehicle and the symptom in front of you. It asks the two or three questions that narrow yours, then names one test with the value to expect, and re-ranks the list on what you measure.
Open your first CaseFree and complete. No account, no card. A Bay Pass after that is $12.00 for 90 days.