U0408: Invalid Data Received From Throttle Actuator A Control Module
What to do
If the car suddenly lost almost all power and will barely rev, that is the safety system working, not the engine breaking. On a modern petrol engine the pedal is not connected to the throttle by a cable — several sensors watch both, and they are constantly checked against each other. When those checks stop agreeing, the car limits itself to little more than idle, because a throttle it cannot verify is one it must not trust.
It is safe in the sense that it will not accelerate on its own, but it is not safe to drive on fast roads in that state — it cannot keep up with traffic. Get it recovered or driven very locally.
Two things worth asking for before any part is bought. Has the throttle been cleaned or replaced, or has the battery been disconnected? If so it may just need its positions relearned — a few minutes with a scan tool, and until it is done the readings genuinely will not match. And has the accelerator pedal's connector been checked? It sits in the footwell, which is the wettest part of the car, and a damp connector there affects several sensors at once.
What this code means
The throttle actuator control module is broadcasting data that cannot be accepted as true. See U0400 for the family's framing.
Throttle by wire is the most heavily supervised system on a petrol engine, and this code sits inside that supervision. There is no cable: the pedal is read by two or three independent sensors, the throttle plate's position is read by two more, and every one of those is cross-checked against the others precisely so that no single failure can open a throttle nobody asked to open.
An implausible throttle report is therefore not tolerated for a moment. The response is immediate and severe — the engine drops to a limited power mode, often refusing to rev beyond a fast idle regardless of the pedal. The corpus describes the same protection at P2138.
That severity is the point, and it is worth saying to a driver who has just experienced it as a frightening loss of power on a road: the system did exactly what it should. A throttle whose position cannot be verified is a throttle that must not be trusted, and reducing the engine to a walking pace is the safe outcome rather than a failure.
Following the family's rule, look upstream. The commonest real causes are the throttle position sensors within the actuator, the pedal sensors, and the wiring between — and the corpus already names the specific traps: a footwell that is one of the wettest places in the vehicle, where the pedal's shared connector sits, and carbon in the throttle bore making the plate's movement disagree with what the motor was told.
A relearn is worth asking about before parts are considered. A throttle body cleaned or replaced, or a module that lost power, needs its closed and open positions relearned — and until that is done the reported position genuinely does not match reality, which produces exactly this code on hardware that is perfectly good.
Likely causes
Start here
- Faulty throttle position sensor · moderate
- Water ingress into the module connector · moderate
- Throttle or pedal relearn procedure not carried out · specialist
- Corroded or loose connector · easy
Less common
- Carbon build-up in the throttle bore · moderate
- Damaged data bus wiring · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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