The battery, fuel trim and acceleration checks

The Home screen has three checks alongside readiness. Each takes a set of live readings and turns them into an answer, which is the part that takes experience to do by eye.

Battery check

Open Battery check from Home. It reads the voltage the engine computer sees, works out whether the engine is running, and tells you whether the number is reasonable.

Engine off, you are measuring the battery at rest. A healthy one reads around 12.6 volts.

Engine running, you are measuring the charging system, and the healthy range is wider than people expect.

A low charging figure is often completely normal. Many newer vehicles deliberately dial charging back to somewhere between 12.6 and 13.2 volts under light load, to save fuel. Run the check again with the headlights and the blower on. If the voltage climbs once there is a load, nothing is wrong. If it stays low with the load on, the alternator, its belt or a connection is worth having looked at.

One limit worth stating: this is a voltage reading, not a load test. A battery can read a healthy 12.6 volts and still fail to turn the engine over on a cold morning. Voltage tells you about charging. Only a load test tells you about capacity.

Fuel trim check

Fuel trim is how far the engine computer is having to correct the fuel mixture away from its baseline. It is the single most informative number in generic OBD-II, and the guide on reading fuel trims goes deeper than this chapter does.

The check needs the engine in a particular state, so it waits rather than giving you a wrong answer. It will tell you which condition it is waiting for: the engine to start, the engine to warm up, the system to enter closed loop, or the engine speed to settle.

Once conditions are met it measures short and long term trim for each bank, first at idle and then at raised engine speed. Comparing those two is the point. A lean condition that is bad at idle and clears up at speed points at a vacuum leak. One that is present at both points somewhere else, often fuelling. Codes like P0171 and P0174 are this measurement crossing a threshold.

The screen ends with what usually causes the pattern it found.

Acceleration test

Acceleration times a run and reports the intervals.

It gates itself before it starts. A safety screen comes up first and you have to acknowledge it, and the app checks the link is fast enough to time anything meaningfully before it begins.

The obvious point, said once: this needs a road where accelerating hard is legal and safe, and somebody other than the driver holding the phone. Set it to metric or imperial in Settings.

Accuracy depends on how quickly your car reports vehicle speed, which varies. Treat the result as a consistent number to compare against your own previous runs, not as a figure to quote against a manufacturer’s.

What to do next

Chapter eleven is getting any of this out of the app and into someone else’s hands.