P2784: Input/Turbine Speed Sensor A/B Correlation
What to do
Drivable with care — expect harsher shifts or limp mode. The transmission has lost a measurement it cross-checks, not a mechanical part, but it may protect itself by simplifying its behaviour. Have live data read before any parts are replaced; with two sensors disagreeing, guessing buys the wrong sensor half the time.
What this code means
Input/turbine speed sensors A and B disagree with each other.
Two sensors watch the input side of the gearbox, and their stories no longer match. Speed sensors are how the module knows what the transmission is actually doing — input speed against output speed is the gear you are really in, measured rather than assumed. Where a design carries two input sensors, it cross-checks them constantly, and this code is the cross-check failing.
A correlation code names the disagreement, not the culprit. Either sensor could be the liar, and the module cannot tell from inside — so neither can a parts cannon. The honest diagnosis is live data: watch both signals on a scan tool while driving, and the one that drops out, spikes or flatlines while its partner stays smooth is the one to pursue.
Metal debris is the transmission-specific twist. These are magnetic sensors, and a wearing gearbox sheds ferrous particles into the very fluid that washes over them. Debris on a sensor tip blurs its reading — so a correlation fault can be the first, cheap notice that something mechanical elsewhere in the box has started to wear.
Losing a trustworthy input speed degrades everything built on it. Shift timing, converter lock-up and slip detection all lean on this measurement, which is why the module often responds with harsher shifts or limp mode even though the shafts themselves are fine.
Likely causes
Start here
- Failed transmission speed sensor · specialist
- Metal debris on the sensor tip · specialist
- Corroded or loose connector · specialist
Less common
- Damaged wiring harness · specialist
Rare
- Faulty transmission control module · specialist
Symptoms
Components involved
Information, not instruction
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Community experience
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