OBD-II Diagnostic Trouble Code
P2784 — Input/Turbine Speed Sensor A/B Correlation
On transmissions with two input or turbine speed sensors, the TCM compared speed sensors A and B and found they disagree more than allowed. Since the TCM cannot trust input speed, it may limit shifting.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Input/Turbine Speed Sensor A/B Correlation
Code
P2784
Category
Powertrain
Subsystem
Transmission
Truck-relevant
Yes
What this means for truckers
P2784 (Input/Turbine Speed Sensor A/B Correlation) — the two speed sensors on the input/turbine side disagree. One of them is wrong. Diagnose by comparing readings under load — replace whichever is off-spec.
Before you replace parts — try this first
P2784 on the Transmission system is most often failed input or turbine speed sensor — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Harsh or erratic shifts, a limp mode with a fixed gear, a check-transmission or check-engine light, and possible speedometer issues on some models.
Common causes
- CommonFailed input or turbine speed sensor ($150–$500)
- CommonWiring or connector fault on one sensor circuit ($80–$350)
- OccasionalMetal debris on the sensor tip ($150–$500)
- RareDamaged tone wheel or internal transmission wear ($1500–$5000)
How to fix
Watch both sensor speeds on a scan tool while driving and see which one drops out
When to use: First step
Check sensor resistance, wiring and connectors
When to use: When one sensor reads erratic
Remove and inspect the sensor for debris, and replace it
When to use: When the wiring is good
Inspect the transmission for internal damage
When to use: When new sensors do not fix the problem
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
Frequently Asked Questions
Can I drive with code P2784?
Drive with caution. P2784 is severity 4 — stop within ~50 miles. Most commercial diesel codes at this severity (turbo, fuel rail, transmission solenoid) can damage adjacent expensive components if left unrepaired. Get to a service location before highway-speed loads.
How much does it cost to fix P2784 on a commercial truck?
Total repair cost ranges $80–$5000 depending on the root cause. The cheapest fix is usually the first one listed below — Failed input or turbine speed sensor — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P2784 fail a DOT inspection?
P2784 typically doesn't fail an inspection on its own. Inspectors focus on safety items (brakes, steering, tires, lights, HOS). Transmission codes only become an issue if the truck has shifting problems severe enough to qualify as 'mechanical fitness' OOS. Fix to prevent escalation.
What's the difference between P2784 and related codes?
P2784 is closely related to: P2789 (Transmission Friction Element Adaptive Learning at Limit). These codes share the same subsystem (transmission) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P2784 is set without the companions, the issue is more isolated.
How do I prevent P2784 from coming back after a repair?
For P2784 (Input/Turbine Speed Sensor A/B Correlation), prevention centers on the transmission system. Service transmission fluid at the OEM interval (typically 150K–250K miles for HD autos). Don't skip the magnet/strainer inspection. After repair, perform an adaptive shift relearn with a scan tool (or drive 40+ miles for adaptive ECMs to relearn naturally).