OBD-II Diagnostic Trouble Code
P0731 — Gear 1 Incorrect Ratio
P0731 means the TCM detected an incorrect gear ratio in 1st gear, indicating the input-to-output speed ratio doesn't match 1st gear. It usually means slipping in 1st.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Gear 1 Incorrect Ratio
Code
P0731
Category
Powertrain
Subsystem
Transmission
Truck-relevant
Yes
What this means for truckers
P0731 (Gear 1 Incorrect Ratio) on an Allison 3000/4000-series HD automatic. The TCM saw input vs output speeds that don't match expected first gear ratio. Causes: worn first-gear clutch pack ($1,800-4,500 trans rebuild), bad input or output speed sensor (always test these first — $200-400), or low transmission fluid. Don't assume rebuild — pull TCM-specific codes and verify fluid level/condition before estimating cost.
Before you replace parts — try this first
P0731 on the Transmission system is most often low or degraded fluid — 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
Slipping or flare on takeoff, poor launch especially loaded, delayed engagement, limp mode, and the check-engine light.
Common causes
- CommonLow or degraded fluid ($100–$600)
- OccasionalFaulty speed sensor ($150–$500)
- OccasionalFaulty shift solenoid or valve body ($300–$1500)
- OccasionalWorn clutch (or band on some designs) applied in 1st gear ($3000–$10000)
How to fix
Check fluid
When to use: First
Verify speed sensor data
When to use: Before suspecting internal failure
Pressure test 1st gear circuit
When to use: When fluid and sensors are good
Overhaul the transmission
When to use: When clutch failure is confirmed
Affects most
- RAM 35006 complaints mention this code
- RAM 55001 complaint mention this code
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0700 — Transmission Control System Malfunction
- → P0701 — Transmission Control System Range/Performance
- → P0702 — Transmission Control System Electrical
- → P0703 — Brake Switch B Circuit
- → P0705 — Transmission Range Sensor Circuit Malfunction
- → Complaints on RAM 3500
- → Complaints on RAM 5500
Frequently Asked Questions
Can I drive with code P0731?
Drive with caution. P0731 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 P0731 on a commercial truck?
Total repair cost ranges $100–$10000 depending on the root cause. The cheapest fix is usually the first one listed below — Low or degraded fluid — 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 P0731 fail a DOT inspection?
P0731 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 P0731 and related codes?
P0731 is closely related to: P0700 (Transmission Control System Malfunction), P0701 (Transmission Control System Range/Performance), P0702 (Transmission Control System Electrical), P0703 (Brake Switch B Circuit). 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 P0731 is set without the companions, the issue is more isolated.
How do I prevent P0731 from coming back after a repair?
For P0731 (Gear 1 Incorrect Ratio), 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).