U300C: Ignition Input Off/On/Start

The ignition input for off, on and start is reporting a state that cannot be accepted as true. It is the signal that distinguishes a car switched off from one running or cranking.

This range is different from every other network family, and the difference decides how it should be diagnosed.

U01xx and U02xx say a module went silent. U04xx and U05xx say a module is talking nonsense about something it watches. This range says a module is reporting a problem with its own foundations — its supply, its earth, or the ignition state that tells it whether the car is even meant to be awake.

Which makes these codes upstream of almost everything else. A module with a marginal earth does not fail cleanly; it reports implausible values about every sensor it reads, sets codes in unrelated systems, and drops off the bus intermittently. A scan tool showing thirty codes across five systems very often has one of these underneath it.

So read this family first and fix it first. Clearing a page of downstream codes before the supply fault is repaired produces a page of downstream codes again by the end of the week.

And start with the battery, genuinely. Not as a formality — a twelve-volt battery weak enough to sag during cranking, or a terminal with corrosion under a clamp that looks tight, reproduces this entire family exactly. It is the cheapest thing to eliminate and by some distance the most common cause.

Ignition state is a broadcast, not a wire to each module, and understanding that changes what this code means.

On any modern car the ignition switch — or the start button and its module — tells the network which state the car is in: off, accessory, on, or cranking. Modules use it to decide whether to wake, whether to power their outputs, and whether to shut down cleanly. They do not each read the key.

So an implausible ignition report is a whole-vehicle event. Expect symptoms that make no sense together: accessories dying at the moment of cranking, modules staying awake after the car is locked, a radio that will not switch off, or a car that behaves as though it is half-on.

The physical causes concentrate in two places. The switch or button assembly itself, which is a mechanical contact set on the highest-cycle control in the car; and the wiring at the column, which flexes with steering adjustment and is disturbed by every airbag, clockspring or column repair.

On push-button cars there is a third: a fob the car can no longer hear reliably, which produces the same ambiguity from the other end.

Why the standard has so many of these codes is worth a line, because a scan tool listing several at once looks alarming. Each position is reported separately so that a module can tell the difference between "accessory" and "on" and "cranking" — states that permit very different behaviour. Several stored together usually means one switch or one connector, not several faults.

The crank-position signal deserves particular attention, since it is the one that tells modules to expect the voltage collapse of starting. A car that sets codes across several systems every time it starts is often reporting that this signal did not arrive, and every module treated a normal cranking dip as a fault.