OBD2 live data and its normal values

At a glance

Each car only gives some of these values. PIDs 00, 20, 40 and so on aren’t data: they’re the list of the ones your car supports, and the first thing any app asks for.

Calculated load (04)

How hard the engine is working compared with what it could give at those revs, as a percentage. Warm and at idle it’s usually between 15 and 35 %, and at full throttle it gets close to 100 %. High load at idle can come from a big consumer (the air conditioning) or from an engine with internal friction; load that never rises at full throttle points to it being short of air.

Coolant temperature (05)

The most useful value of all. In normal running it should be between 85 and 105 °C; some engines with an electronic thermostat run at 105 or 110 °C under light load, and that’s normal. If it takes a long time to get past 70 or 80 °C, suspect the thermostat (code P0128). Above 110 to 115 °C for any length of time, stop and have a look.

Fuel trims (06, 07, 08 and 09)

They say how much the control unit is correcting the amount of fuel to get the mixture just right. Short term trim moves around 0 all the time; long term trim is what it has learned over time. Added together, a healthy engine sits between -10 and +10 %.

• Positive, above +10 %: the control unit is adding fuel because it sees the mixture as lean. It’s usually air getting in where it shouldn’t (a cracked vacuum hose, an intake gasket), a dirty mass air flow sensor or low fuel pressure. Those are codes P0171 and P0174.
• Negative, below -10 %: it’s taking fuel away because it sees it as rich. A leaking injector, too much fuel pressure or the purge valve stuck open. Those are codes P0172 and P0175.

Tip: if the trims are very positive at idle and drop when you hold 2,500 rpm, it’s almost certainly a vacuum leak. On diesels these values are hardly ever available.

Manifold pressure (0B)

The absolute pressure in the intake. With the engine off it reads atmospheric pressure, about 100 kPa. On a non turbo petrol engine at idle it drops to 25 to 45 kPa, because the throttle is almost closed; on a diesel it stays close to atmospheric. On turbo engines it goes above 100 kPa under acceleration: the extra is the boost pressure. High pressure at idle on a petrol engine is usually a vacuum leak or low compression.

Engine speed (0C) and vehicle speed (0D)

Engine revs and road speed. Warm and at idle, between 650 and 900 rpm; cold, for the first few minutes, it can sit at 1,000 or 1,200. OBD speed is usually a little higher than your real speed but lower than the speedometer’s, which by law must never read less than you’re doing.

Ignition timing advance (0E)

How many degrees before top dead centre the spark fires. At idle, between 5 and 15 degrees; cold, just after starting, it can be negative to warm the catalyst up faster. If the advance drops suddenly and often under load, the control unit is pulling timing because of knock: poorer quality fuel, carbon build-up or a knock sensor.

Intake air temperature (0F)

On the move, the outside temperature plus a few degrees; stuck in traffic it can climb 20 or 30 degrees from engine heat. On turbo engines, if it’s measured after the intercooler, it rises when you work the engine hard. If it reads -40 °C, it’s almost always a disconnected sensor.

Mass air flow (10)

The grams of air per second measured by the mass air flow sensor. At idle, roughly between 2 and 5 g/s on a 1.4 to 2 litre engine; at full throttle it rises a lot, and as a very rough rule a petrol engine’s maximum g/s is around 0.8 times its horsepower. Low flow across the whole range points to a dirty or worn sensor.

Throttle (11) and pedal (49 and 4A)

The position of the throttle and of the accelerator pedal, as a percentage. With an electronic throttle, at idle the throttle doesn’t read 0 % but a small opening, and even flat out it may not reach 100 %. What matters is that it rises and falls smoothly and that the pedal’s two tracks move together.

Oxygen sensors (14 to 1B and 24 to 3B)

Oxygen sensors measure the oxygen in the exhaust gas. The voltage type, the traditional kind, switch between 0.1 and 0.9 V several times a second before the catalyst with the engine warm; if they sit still or respond very slowly, the sensor is old. The one after the catalyst should be steady, between 0.5 and 0.8 V: if it copies the front one, the catalyst isn’t cleaning any more (P0420).

Wideband sensors, on more modern cars, give lambda directly: 1.00 is the ideal mixture, below that is rich and above it is lean. On a petrol engine running steadily it stays glued to 1; on a diesel it’s normally very lean, from 1.5 to over 4 at idle.

Distance with the light on and since clearing (21, 30 and 31)

PID 21 counts the distance driven with the check engine light on; 31, the distance since the fault codes were cleared, and 30, how many times the engine has warmed up since then. If you’re buying a used car and 31 shows only a few kilometres, someone has cleared the codes recently: ask why. The ITV looks at 21.

Fuel rail pressure (22 and 23)

The petrol or diesel pressure in the injector rail. On a common rail diesel it goes from about 250 to 400 bar at idle to over 1,600 bar at full throttle; on a direct injection petrol engine, from about 40 bar at idle to 150 or more under load. If it doesn’t reach what the control unit asks for, codes P0087 or P0093 are set.

Barometric pressure (33)

About 101 kPa at sea level, and it drops with altitude: about 93 kPa at 700 metres, as in Madrid or Valladolid. If it doesn’t match where you are, the sensor is lying, and the control unit gets the mixture and the boost wrong.

Control module voltage (42)

The voltage reaching the control unit, which is almost the same as the battery’s. With the engine running, between 13.5 and 14.8 V; with a smart alternator it can drop to 12.5 V or rise to 15 V depending on what suits the car. With the engine off, a healthy battery at rest gives between 12.4 and 12.7 V; below 12.2 V it’s low.

Commanded lambda (44)

The mixture the control unit wants at that moment: 1.00 on a petrol engine running steadily, below 1 (richer) at full throttle or to protect the catalyst, and above 1 on some lean burn engines.

Oil temperature (5C)

Not every car gives it over standard OBD. Warm, between 90 and 120 °C, depending on how hard you’re driving. It’s the one to watch before pushing the engine: until the oil gets past 80 or 90 °C, better not ask for everything it has.

Fuel rate (5E) and odometer (A6)

PID 5E gives the litres per hour the engine is burning; at idle, between 0.5 and 1 l/h on an ordinary car. Not every car gives it. PID A6, the odometer, is only on recent cars, roughly from 2019; on the rest, the mileage has to be read from each control unit.

With Final OBD

The gauges in Free show live whatever standard OBD data your car gives, with five themes to choose from. With Pro you also get your make’s own data that standard OBD doesn’t provide, charts and your own dashboards. How many readings per second you see depends on the adapter: it’s all in the adapter guide.