Reading live data: what’s normal
Live data shows what the engine computer sees right now. The rough ranges below help you spot a reading that’s clearly off. They’re general guidance: your car’s manual or service information has the values for your engine.
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Getting started
- Connect Read My Car to your car, or pick a demo car, and open Live.
- Choose the readings you want. Only the ones your car reports are offered.
- Let the engine warm up fully before judging the values; many readings differ on a cold engine.
- Record a drive to a CSV file if you want to look at it later, or show it to a mechanic.
Read My Car marks a few readings that are clearly outside normal operation on nearly every car. Everything else is up to you to judge, with the guidance below.
Coolant temperature
Once warm, most engines settle somewhere around 90 °C. The exact value depends on the engine and the conditions: some run hotter by design, and it can rise in slow traffic until the cooling fan starts. Read My Car marks readings above 112 °C as very hot.
If the temperature stays well below its usual value after a longer drive, the thermostat may be stuck open. If it climbs towards the red, stop when it’s safe to do so and switch the engine off.
Fuel trim
Fuel trim is how much the engine computer corrects the fuel it injects. Short term fuel trim changes from moment to moment; long term fuel trim is the correction it has learnt. Positive values mean it’s adding fuel, negative values that it’s taking fuel away.
As a general guide, each stays within about ±10 % on a healthy engine, and short term trim moves around zero. Read My Car marks values beyond ±10 % as lean or rich. Look at the two together: a large long term value means the computer has been correcting for a while.
Compare idle with a steady higher engine speed. A lean reading at idle that gets better at higher speed often points to a vacuum leak.
Oxygen sensors
On a warm petrol engine, a conventional oxygen sensor in front of the catalytic converter keeps switching between roughly 0.1 and 0.9 V, about once a second or faster, and faster at higher engine speed. The sensor behind a working converter gives a much steadier signal.
Many newer cars have a wide-range front sensor instead. It reports differently and doesn’t switch like that, so don’t judge it by the same numbers. Diesels don’t show the switching either.
- P0133: oxygen sensor slow response
- P0420: catalyst efficiency below threshold
- Oxygen sensors explained
Air flow (MAF) at idle
A rule of thumb often quoted for a warm petrol engine: at idle, the air flow in g/s is roughly the engine size in litres, so about 2 g/s for a 2-litre engine. It varies a lot between engines, so treat it only as a sanity check; the service information for your car has the real value. A reading far below it can point to a dirty sensor or an air leak after it.
Battery voltage
- Engine off
- A healthy, fully charged battery that has rested for a while reads roughly 12.6 to 12.8 V; around 12.4 V or less means it needs charging, and clearly lower that it’s flat or weak.
- Engine running
- On many cars roughly 13.5 to 14.5 V. Cars with smart charging or start-stop can show less at times, so a single low reading isn’t proof of a faulty alternator.
Read My Car marks readings below 12 V or above 15 V. Its voltage reading is the supply the engine computer sees, which can differ a little from a meter on the battery.
Questions
Why does my car show fewer readings than the demo?
Read My Car only offers the readings your car reports, and that differs between cars. Older cars often report fewer.
Are these values limits for my car?
No. They’re general guidance that holds for many cars. Your car’s manual or service information has the values for your engine.
Can I read live data while driving?
Yes, but only look at the screen when it’s safe, or let a passenger watch. Recording to a CSV file lets you look at a drive afterwards.