Faulty battery current sensor

The sensor measuring current into and out of the traction battery has failed, drifted, or lost its circuit.

Its importance is out of proportion to its size, because state of charge is counted rather than measured. A battery does not expose how full it is — there is no float and no dipstick, and voltage alone is a poor guide under load. So the module integrates current over time, adding what goes in and subtracting what comes out, and that running total is the charge figure the whole car is managed by.

Which gives this cause an unusual failure mode: it is not wrong, it is cumulatively wrong. A sensor reading a few per cent high does not produce an obviously bad number. It produces a small error added to a total on every sample, and the total drifts steadily away from the truth with nothing to contradict it until a full charge or a deep discharge forces a recalibration. That is why packs carry two of these and compare them — a single sensor drifting is undetectable, and two disagreeing is not.

The symptoms are never about current, which is why this cause gets missed. What the driver sees is a charge display that jumps, an electric range that no longer matches reality, a hybrid that stops using its battery, or a pack that appears to fill in minutes. All of those read as a failing battery, and a failing battery is the expensive answer.

So the useful rule is to eliminate the measurement before accepting a pack quote. The sensor is a fraction of the price and produces the same complaints. Note also that a correlation code between two sensors reports a disagreement rather than a verdict: it cannot say which one is lying, and resolving that needs a third opinion rather than a guess.