Reading live data: what good sensor values look like, and what they mean when they are not

A fault code tells you which test failed. Live data tells you what the engine is actually doing, and it is the difference between replacing the part a code names and finding the fault that set it.

This was the second most-read article on the site it came from, and the reason is that it does something most diagnostic writing does not: it gives expected values, and it says what a wrong one means.

The idea worth taking from it

Before any list of numbers, the original makes an observation that is better than the numbers:

Parameters are linked in chains, and diagnosis means recognising the chain rather than reading one value.

A single parameter out of range rarely tells you much, because most of them are calculated from others or corrected by others. Fuel trim looks wrong when the MAP sensor is lying. Ignition timing looks wrong when the coolant temperature sensor reads cold. The value you are staring at is often the symptom of a value three steps upstream.

So when idle is unstable, the original says to look at these together — idle valve position, throttle angle, manifold pressure, ignition timing, injection duration — because the pattern across them identifies which end of the chain is at fault in a way that no single one does.

That habit is the whole skill. The tables below are only useful inside it.

Engine speed

A crankshaft sensor failing outright gives no start at all, because the ECU will not fire injectors or coils without knowing where the engine is. Failing intermittently gives cut-outs, a tachometer that sticks, or a rev limit that arrives early.

Check the specified idle for the actual car rather than assuming. The original quotes 850 rpm for Peugeots, which is right for those and not for everything.

Battery and charging voltage

The original says battery voltage with the engine running should be 12 to 14 volts. That is wrong, and it matters more than most errors in this archive because it would let a car that is not charging at all pass as normal.

A reading of 12.4 V with the engine running is not the bottom of a normal range. It is a charging system that has stopped working. The original's own next sentence — that a rise with throttle points at the alternator — is sound, and usually means worn brushes.

The threshold for "too high" is also stated too tightly. Above 14 V is ordinary, especially on a cold morning; above 15 V is the number worth acting on. See the alternator and charging system for the full picture and the ripple test, which finds a failing rectifier that voltage alone reads as healthy.

Throttle position

A throttle angle that does not read zero at idle throws off everything downstream, because the ECU uses closed-throttle as the signal to run idle control at all. A stuck-open stop screw, a carbon build-up around the plate, or a sensor out of adjustment all produce it — and the resulting symptom is usually blamed on the idle valve.

The angle you see is calculated from the sensor's voltage, so where the tool offers both, watch the voltage when hunting for a dead spot: it should sweep smoothly across the whole travel, and any step or dropout is a worn track.

Coolant temperature

This is the parameter most worth checking first on any driveability complaint, because so much is corrected by it — fuelling, timing, idle target, fan control, and whether closed-loop operation starts at all. A car that never reaches temperature may also simply have a thermostat stuck open; see the engine thermostat.

Manifold pressure and load

The key-on check is the best single test of a MAP sensor and it costs nothing — details in the MAP sensor.

Calculated load is derived largely from this on a speed-density system, so it is a useful cross-check: low at idle, high under load, and a value that barely moves says the same thing a stuck pressure reading does.

Fuel trim — the parameter that finds faults nothing else names

The original does not cover this and it is the most diagnostic number on the tool.

Short-term trim is the ECU's live correction from the oxygen sensor. Long-term trim is what it has learned it must apply permanently. Both are percentages, and both should sit near zero on a healthy engine.

• Both strongly positive — the ECU is adding fuel, so it thinks the mixture is lean. An unmetered air leak, a weak fuel pump, dirty injectors, or a MAF or MAP reading low.
• Both strongly negative — it is taking fuel away. Excessive fuel pressure, leaking injectors, or a sensor reading high.
• Positive at idle, normal at speed — a vacuum leak, because a fixed extra airflow matters proportionally far more at idle.
• Normal at idle, positive at speed — a fuel delivery limit that only appears under demand.

That last pair is why trims are worth reading at more than one engine speed. A lean code plus large positive trims means the oxygen sensor is telling the truth, and the sensor is the last thing to replace.

Testing actuators

Most scan tools will also command outputs — run the fuel pump, cycle the idle valve, fire an injector, switch a relay. This turns a passive reading into a test: if the tool commands a component and nothing happens, the fault is between the ECU and the part, and you have just eliminated the ECU's decision-making from the question.

A worked chain

Unstable idle, no unusual noises, no mechanical suspicion. Read together:

1. Idle valve position — if it is being held unusually far closed to reach a correct idle speed, the engine is getting air from somewhere else. That is a leak.
2. Throttle angle — if it does not read closed, the ECU may not be running idle control at all.
3. Manifold pressure — steady and plausible, or wandering with the speed?
4. Fuel trims — positive says lean, which supports the leak; near zero says the air is being measured correctly and the problem is elsewhere.
5. Ignition timing — swinging widely is the ECU chasing the speed rather than causing the problem.

No one of those five identifies the fault. The pattern across them does, which is the point the original makes and the reason it was worth translating.

Video guides

Video, for the parts of this that are easier watched than read.