Diagnose & Clear P0720 with iCarsoft CR MAX BT: Fix Output Speed Sensor Circuit Malfunction

If your check engine light illuminates and a scan returns P0720, your vehicle’s transmission system is signaling a problem with the Output Speed Sensor (OSS). This OBD-II code stands for "Output Speed Sensor Circuit Malfunction"—indicating the Transmission Control Module (TCM) or Engine Control Module (ECM) has detected an abnormal voltage, no signal, or intermittent communication from the sensor that monitors the rotational speed of the transmission’s output shaft.

The OSS is critical for smooth transmission operation: it works with the Input Speed Sensor (ISS) to calculate gear ratios, enabling the TCM to adjust shift timing, hydraulic pressure, and torque converter lockup. When P0720 occurs, this speed data is lost or corrupted—leading to erratic shifting, torque converter issues, transmission overheating, or limp mode (to prevent damage).

Basic scanners may only flag "speed sensor fault" but can’t compare input/output speeds or analyze signal patterns. The iCarsoft CR MAX BT—with its wireless dual-speed monitoring, waveform analysis, and transmission pressure tests—solves this. Let’s explore how to diagnose and resolve P0720 using this advanced tool.

iCarsoft CR MAX BT displaying live OSS/ISS speeds, gear ratio data, and transmission fluid temperature for P0720 diagnosis

Understanding P0720: Causes & Key Symptoms

A faulty OSS disrupts transmission speed data, leading to distinct performance and safety issues:

Key Symptoms of P0720

• Check Engine Light: Illuminates when the TCM/ECM detects no signal, erratic voltage, or a short/open OSS circuit for 2+ drive cycles.
• Erratic Shifting: Transmission shifts too early/late, slips between gears, or refuses to shift—caused by missing speed-based shift cues.
• Torque Converter Problems: Converter fails to lock up (reduces MPG by 5–10%) or locks up unexpectedly (causes shuddering at low speeds).
• Transmission Overheating: Incorrect shift timing increases friction, raising fluid temperature (look for a temperature warning light or burning smell).
• Limp Mode Activation: TCM limits vehicle to 2nd/3rd gear (30–40 mph max) to prevent internal transmission damage.
• Speedometer Issues: In vehicles relying on OSS data, the speedometer shows incorrect speeds, drops to zero, or fluctuates unpredictably.

Common Causes of P0720

Why iCarsoft CR MAX BT Excels at Diagnosing P0720

The CR MAX BT outperforms basic tools with OSS-specific diagnostics to pinpoint P0720’s root cause:

Step-by-Step: Diagnose P0720 with iCarsoft CR MAX BT

1. 1. Connect & Confirm the Code

   Plug the CR MAX BT into your vehicle’s OBD-II port and pair it with your smartphone/tablet via the iCarsoft app (Bluetooth setup: 30 seconds).
   Select your vehicle using Auto VIN Scan (instant VIN read) or manual entry (make/model/year/transmission type: automatic/CVT).
   Navigate to Transmission > Fault Codes > Read Codes to confirm P0720. Tap Code Details for vehicle-specific insights (e.g., "Toyota: OSS Signal = No Detection – Output Shaft Speed = 0 RPM").

2. 2. Locate the Output Speed Sensor

   Use the CR MAX BT to avoid misidentification:
   
   Go to Component Location > Transmission > Sensors > Output Speed Sensor.
   The app displays a 3D diagram: The OSS is typically mounted on the side or rear of the transmission case (near where the driveshaft connects), with a 2–3 wire connector (power, ground, signal). For front-wheel-drive vehicles, it may be on the transaxle.

3. 3. Analyze Live Speed & Gear Ratio Data

   The CR MAX BT’s real-time data reveals OSS discrepancies:
   
   Start the engine, shift to Drive, and drive at 10–20 mph (safe, low-speed conditions).
   In the app, go to Transmission > Live Data > Speed Sensors and monitor:
   - "Output Shaft Speed (OSS)": Should match vehicle speed (e.g., 20 mph = ~800 RPM, varies by tire size). P0720 = 0 RPM or erratic spikes.
   - "Input Shaft Speed (ISS)": Should show consistent rotation (e.g., 2,500 RPM at 20 mph in 1st gear).
   - "Calculated Gear Ratio": Derived from ISS/OSS (e.g., 3:1 in 1st gear). A ratio of "Infinite" = no OSS signal.
   
   Compare to speedometer: If the speedometer works but OSS reads 0, the OSS or its circuit is faulty (not the tone ring).

4. 4. Inspect the Sensor, Connector & Tone Ring

   Physical damage or contamination is often the cause—check with the CR MAX BT’s guidance:
   
   Sensor Inspection:
   Locate the OSS using the app’s diagram. Disconnect the connector and remove the sensor (use a 10–14mm socket). Check for:
   - Metal shavings on the sensor tip (sign of internal transmission wear—requires fluid replacement).
   - Cracks in the sensor housing (from impact or overheating).
   - Oil contamination (sign of transmission fluid leaks—clean with brake cleaner if minor).
   
   Tone Ring Check:
   Look through the empty sensor port at the tone ring (a toothed gear on the output shaft). Check for cracked teeth, rust, or debris—use a flashlight for visibility. A damaged ring needs professional replacement.
   
   Connector Check:
   Examine the plug for corrosion (white/green deposits), bent pins, or water intrusion. Clean terminals with electrical contact cleaner; straighten bent pins with needle-nose pliers.

5. 5. Test the OSS Circuit’s Electrical Function

   Validate OSS and circuit health with the CR MAX BT—test type depends on sensor design:
   
   Voltage Supply Test (All Sensors):
   Turn the ignition to "On" (engine off). Use the app’s Multimeter Function to measure voltage at the OSS’s power pin (refer to the app’s OSS Pinout Guide). Normal = 5V or 12V (varies by vehicle). 0V = check TCM fuse (labeled "TCM" or "TRANS") or wiring to the TCM.
   
   Resistance Test (Magnetic Sensors, 2-wire):
   Disconnect the OSS from the harness. Measure resistance across the signal and ground pins. Normal = 500–1,500 ohms (check app’s specs). Infinite (open) or 0 ohms (short) = replace the sensor.
   
   AC Voltage Test (Active Sensors, 3-wire):
   Reconnect the OSS connector. Start the engine, shift to Park, and rev to 1,500 RPM. Measure AC voltage at the signal pin (via the connector). Normal = 0.5–1V AC (increases with RPM). 0V = faulty sensor or tone ring.

6. 6. Diagnose Wiring for Shorts & Open Circuits

   Damaged wires disrupt OSS signals—test with the CR MAX BT:
   
   Short Circuit Test:
   Navigate to Special Functions > Electrical Tests > Short Circuit Test.
   Connect one test lead to the OSS’s signal wire (at the connector) and the other to a clean ground (e.g., battery negative). A reading <5 ohms = shorted wire—repair with heat-shrink butt connectors.
   
   Continuity Test:
   Check continuity between the OSS’s connector and the TCM (use the app’s wiring diagram). Resistance >1 ohm = broken wire—repair or replace the harness (pay attention to wires near hot exhaust components).

7. 7. Check Transmission Fluid & Temperature

   Low or overheated fluid often triggers P0720—verify with the CR MAX BT:
   
   Navigate to Transmission > Live Data > Fluid Status.
   - Fluid Temperature: Normal = 175–225°F (79–107°C). >250°F (121°C) = overheating—stop driving and let it cool; address leaks or low fluid before replacing the OSS.
   - Fluid Level/Condition: Use the app’s Fluid Check Guide (under Maintenance) to inspect level (follow manufacturer steps: engine running, warm fluid) and condition. Dirty/burnt fluid = drain and replace with OEM-spec fluid.

8. 8. Repair & Clear P0720

   Fix the Root Cause (prioritize based on test results):
   - Replace the OSS (use OEM parts—check the app’s Part Lookup for exact fit).
   - Repair damaged wiring with heat-shrink connectors; replace severely corroded harnesses.
   - Clean/replace the OSS connector; apply dielectric grease to prevent moisture damage.
   - Replace a damaged tone ring (requires professional transmission disassembly—critical for high-mileage vehicles).
   - Change transmission fluid if low, dirty, or contaminated.
   
   Clear the Code: In the app, go to Transmission > Fault Codes > Clear Codes to delete P0720.

9. 9. Validate the Repair

   Confirm safe OSS and transmission operation:
   - Recheck live data: OSS speed must match vehicle speed; gear ratios should be consistent (e.g., 3:1 in 1st, 2:1 in 2nd).
   - Test drive: Drive 20–60 mph (city/highway) to check for smooth shifting, no torque converter shudder, and stable speedometer.
   - Monitor temperature: Ensure fluid temp stays within 175–225°F during driving.
   - Re-scan: Use the CR MAX BT to recheck codes—no P0720 recurrence = successful repair.
   - Verify limp mode: Ensure the vehicle doesn’t enter limp mode during acceleration.

Preventing P0720 Recurrence

The CR MAX BT helps maintain reliable OSS and transmission performance long-term:

• Sensor Maintenance: Use the app’s Service Reminder to inspect the OSS every 60,000 miles and replace it every 100,000–150,000 miles (sensors wear from heat and vibration).
• Fluid Care: Replace transmission fluid every 60,000–100,000 miles (follow the app’s manufacturer-specific schedule) to prevent contamination and sensor damage.
• Connector Protection: Apply dielectric grease to the OSS plug annually to repel moisture and corrosion (critical for vehicles in wet climates).
• Transmission Leak Checks: Inspect for fluid leaks quarterly (use the app’s Leak Detection Guide)—leaks damage the OSS and tone ring.
• Regular Scans: Use the CR MAX BT’s Quick Scan quarterly to compare ISS/OSS speeds and catch early signal irregularities (e.g., intermittent voltage drops).