P0899: Transmission Control System Malfunction Indicator Lamp Request Circuit High
What to do
Nothing will feel wrong — which is precisely the problem, because this fault means future transmission problems will not light the warning lamp either. Worth repairing on that basis alone, even though the car drives perfectly.
What this code means
The transmission module's warning lamp request circuit is held high — an open circuit or a short to supply — so the request never reaches the engine module and the lamp is never illuminated on the transmission's behalf.
This is the silent failure, and the counterpart to P0898 in every respect. Nothing about the vehicle feels wrong, nothing looks wrong, and every future transmission fault will occur without any indication to the driver. It is the same consequence described under P0802 and it closes the transmission block on the same note: the least dramatic faults in this range are the ones that remove your warning that the dramatic ones have happened.
Likely causes
Start here
- Open circuit in module wiring · moderate
- Corroded or loose connector · easy
Less common
- Wiring shorted to voltage · moderate
- Faulty transmission control module · specialist
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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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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