U140900 on Audi: Internal temperature measurement implausible signal (Unauthorized)
Diagnostic trouble code U140900 on Audi vehicles — what it means, the common
causes, and how to confirm the real fault before replacing parts.
What does U140900 mean?
U140900 indicates: Internal temperature measurement implausible signal (Unauthorized).
This relates to the Network / communication system.
Common causes of U140900
Implausible temperature signal from internal temperature sensor
Temperature signal transmitted via LIN message may be corrupted or invalid
Manipulation/tampering of temperature sensor circuit suspected by ECM
What to check (live data)
Engine coolant temperature (must be > 97.5°C / 207.5°F for diagnostics to enable)
Ambient temperature (must be > 9°C for diagnostics to enable)
Radiator fan status (diagnostics disabled for 45 seconds at fan On/Off or Off/On transitions)
Temperature curves/gradients measured during 10-second measurement windows
LIN message signal from temperature sensor (not transmitted as voltage value)
Parts often involved in U140900
Internal temperature sensor
Engine Control Module J623 (ECM/ECU)
LIN message bus/wiring
Don't replace parts until the root cause is confirmed — many of these codes are caused by wiring, a connector or an upstream fault, not the part itself.
Get the confirmed root cause for U140900 — on YOUR vehicle
A code is a symptom, not a diagnosis. AutoDX Pro cross-checks several
independent assessments, pinpoints the actual root cause, gives you the
exact tests to confirm it, and shows the parts — only after the cause is
confirmed. Reads your scan-tool photo and decodes your VIN with open
recalls.
U140900 is a diagnostic trouble code: Internal temperature measurement implausible signal (Unauthorized).
What are the common causes of U140900 on an Audi?
Common causes include: Implausible temperature signal from internal temperature sensor, Temperature signal transmitted via LIN message may be corrupted or invalid, Manipulation/tampering of temperature sensor circuit suspected by ECM. The exact cause depends on the vehicle — confirm it before replacing parts.
Can I keep driving an Audi with U140900?
It depends on the fault and your symptoms. Some causes are minor, others can damage components or affect safety. Get the root cause confirmed before deciding.
How do I fix U140900 on an Audi?
Confirm the actual root cause with targeted tests first, then replace only the part that's at fault. AutoDX Pro gives you the root cause, the exact tests, and the parts for U140900 on your specific Audi.