You can drive, but do not put it off
It stalls at idle
At the lights, or when you lift off, the engine dies. It usually restarts straight away.
You can usually get home or to a garage, but every day of delay adds to the bill.
What to look at in the live data
- Idle speed just before it dies: does it sag smoothly or cut off.
- Fuel trims at idle.
- Air flow: too low points to a dirty throttle body.
- Voltage: a weak alternator drags the revs down when lights and blower come on.
Common causes, cheapest first
- A dirty throttle body.
- An air leak.
- The crankcase ventilation valve, clogged or stuck open.
- A weak alternator or a bad earth.
- A crankshaft position sensor on its way out.
Codes that show up with this
P0505Idle air control system malfunctionP0171System too lean (bank 1) — air leak or not enough fuelP0335Crankshaft position sensor circuit malfunction
These parameters in the reference
Engine RPMShort term fuel trim B1Mass air flow (MAF)Control module voltageShort term fuel trim B2Long term fuel trim B2Relative throttle positionAbsolute manifold pressureCommanded EGR
Lamps that may be on
Do it step by step
Four readings on one screen show what exactly is swinging.Unstable idle — how to find the cause
A symptom is not a diagnosis. The same sign comes from a dozen different causes — the readings are what tell them apart, not the feeling.
How the app helps
CarScan shows exactly these readings live, remembers the highest and the lowest value of the trip, and puts them into a report you can hand to a mechanic.