When a car logs a fault it stores a freeze frame: the speed, the revs, the temperature and the mileage at the moment it happened. This car stores one every few weeks, at fifty eight miles an hour, warm, in gear, at a mileage sixty miles beyond what the odometer reads, and the odometer has not moved.
I am a diagnostic technician, which means I spend my day plugged into cars.
What a freeze frame is.
When the engine controller sees something out of range it logs a fault code, and with it a freeze frame: the road speed, the engine speed, the coolant temperature, the load, the throttle position, and the odometer reading at that moment. It is there so that a technician can see the conditions the fault happened under, and it is honest, because it is written by the car about itself.
An odometer reading in a freeze frame is the odometer reading. They come from the same counter.
One customer's car stores a fault every few weeks, always the same code, a minor one about a sensor, and the freeze frame is always the same shape.
Fifty eight miles an hour. Two thousand one hundred revs. Coolant at ninety. Fourth gear. And an odometer reading that is about sixty miles more than the car's odometer shows.
The car does not do fifty eight. The customer is elderly and drives it to the shops and back, four miles, twice a week, at thirty, and the odometer agrees with her: it moves about eight miles a week and it has for years.
The freeze frame says that between her trips the car has been driven, warm, in fourth, at motorway speed, for a distance that would take it sixty miles past where it is.
And then the odometer does not carry those sixty miles. The next freeze frame is sixty miles beyond the odometer again, not a hundred and twenty. Whatever drive it records, the car does not keep the distance.
What has been checked.
The controller. Replaced, at the customer's expense, on my recommendation, which I regret. The new controller stored a freeze frame in its third week with the same numbers.
The odometer. It is electronic and it reads from the same source the freeze frame does, and I have watched it. It does not move between her trips.
The car's position. She agreed to a tracker, the kind the insurance people fit, for a month. The tracker shows her trips and nothing else. Two freeze frames were stored that month, both on nights the tracker has the car on her drive, stationary, with the ignition off.
The port itself. A freeze frame written by another tool, a prank, a previous garage. The frame's time stamp comes from the car's own clock and the car's own clock is right, and the frames are written at times when the car is parked on a drive with a tracker on it.
What the frame says that I did not notice for a year.
There is a field for ambient temperature, from a sensor in the bumper.
On every frame it reads minus four. In July. On a car parked on a drive in a town where it was eighteen degrees at that hour, according to the tracker's own log, which records the weather because the insurers like to.
Wherever that car is driving at fifty eight in fourth, it is cold there.
What is done about it.
The code is a minor one and I clear it when she comes in, and I have told her it is a sensor that gets confused, which is a sentence I can say with a straight face.
I have not told her about the temperature. She is eighty one and she likes the car and she has said more than once that it seems to like her.
What I do.
I plug in, I read, I clear.
The one thing I have changed is that I read her car in the yard, not in the workshop.
The reason is a reading in the workshop, on the ramp, at the end of a day, with the car up and the engine cold and the port under the dash and me on a creeper underneath reaching up to the tool.
The tool reported a new frame being written as I was connected. Live. Fifty eight miles an hour, two thousand one hundred, ninety degrees, minus four ambient, while the car was on a ramp above my head with its wheels a metre off the floor and its engine cold, and the wheels, all four of them, were turning.