P0720: P0720 Diagnostic Guide

What this code means

P0720 may indicate that the PCM is seeing a problem with the output shaft speed sensor circuit signal.

What the vehicle may do

The vehicle may have transmission shift complaints.
A warning indicator may come on.
The transmission may behave as if speed information is unreliable.

Possible fault areas

Possible connector or terminal problems.
Possible wiring opens or shorts in the OSS circuit.
Possible OSS sensor concern.
Possible PCM-side circuit concern.

Diagnostic path

Opening context for P0720

On this 2020 to 2025 F-250 with the 7.3 gas engine and 10R140 transmission, P0720 points us toward a possible output shaft speed sensor circuit problem. In plain language, the PCM may not be seeing the OSS signal the way it expects to. The truck may have shift complaints, may set a warning indicator, or may behave like the transmission speed information is unreliable. Broadly, we’re thinking about possible connector issues, wiring faults, the OSS sensor circuit, or in some cases the PCM side of the circuit. If other speed-sensor-related codes are present, especially a more specific one, handle that diagnostic direction first instead of guessing.

Know what scan data is normal before testing

Before you condemn a speed sensor from scan data alone, remember that not every zero reading is a fault. The ISSA sensor will show 0 RPM in 1st through 6th gears, and the ISSB sensor will show 0 RPM in 1st and 2nd gears. That matters because this test family covers multiple transmission speed sensors, but for P0720 we’re going to stay on the OSS-related circuit path.

Start at the external harness and check OSS VREF

For this code, stay on the related OSS circuits. Start with the ignition off, disconnect the transmission vehicle harness at C168A, and inspect the connector closely. Look for damaged terminals, pushed-out terminals, corrosion, loose wires, and missing or damaged seals. Then turn the ignition on and check OSS VREF at C1575A Pin 7 to ground. You’re looking for approximately 9 volts. If that voltage is there, move on to the OSS signal return checks. If it is not there, turn the ignition off, disconnect PCM connector C175E, and check C1575A Pin 7 to ground for a short to ground. That resistance should be greater than 10,000 ohms. If it is not, repair the circuit. If it passes, check the VREF circuit for an open from C1575A Pin 7 to C175E Pin 39. That resistance should be less than 3 ohms. If it is not, repair the circuit. If the circuit is good at that point, the directed repair is a new PCM followed by the transmission strategy download.

Check the OSS signal return circuit outside the transmission

Once VREF is present at the first check, move to the OSS signal return circuit. With the ignition off and PCM C175E disconnected, check from C1575A Pin 4 to C175E Pin 115. The resistance should be less than 3 ohms. If it is not, repair the circuit. If it passes, check C175E Pin 115 to ground. That resistance should be greater than 10,000 ohms. If it fails, repair the circuit. If it passes, check whether the OSS signal return is shorted together with voltage circuits by measuring C175E Pin 115 to C175E Pins 23, 24, 39, 60, 80, 93, and 94. Those resistances should be greater than 10,000 ohms. If they are not, repair the circuit. If they pass, the external wiring checks have taken you as far as they can, and the next checks move inside the transmission harness path.

Check the internal transmission harness path

Next, remove the main control valve body and disconnect the OSS sensor connector C1107. Check the internal VREF path from C1107 Pin 2 to the transmission component side, pin 7. That resistance should be less than 3 ohms. If it does not pass, install a new transmission case wiring harness. If it passes, check the transmission component side, pin 7 to ground. That resistance should be greater than 10,000 ohms. If it fails, install a new transmission case wiring harness. Then check the OSS signal return inside the transmission from C1107 Pin 1 to the transmission component side, pin 4. That resistance should be less than 3 ohms. If it does not pass, install a new transmission case wiring harness. If it passes, check C1107 Pin 1 to ground. That resistance should be greater than 10,000 ohms. If it fails, install a new transmission case wiring harness.

Reconnect the transmission and finish at the OSS sensor

After the internal harness checks pass, reconnect the transmission vehicle harness at C1575A. Now check the OSS signal return with the transmission connected, from C1107 Pin 1 to C175E Pin 115. The resistance should be less than 3 ohms. If it is not, treat it as a terminal fitment issue and repair the vehicle wiring harness. If it passes, connect PCM C1750, turn the ignition on, and check VREF at the OSS sensor from C1107 Pin 2 to ground. The voltage should be 8.0 to 9.2 volts. If it is not in that range, treat it as a terminal fitment issue and repair the vehicle wiring harness. If it is in range, the directed repair is to install a new OSS sensor.

Keep related ratio faults in their own lane

After the P0720 path, if you are also working one of the related gear-ratio faults, keep that as a separate confirmation path. In order, those related checks use the same basic move: ignition on, clear that specific DTC from the PCM, road test the truck, then retrieve all PCM continuous memory codes again. Do that for each related gear-ratio fault you are actually chasing, rather than blending those results into the OSS circuit decision. If the same ratio fault returns, continue into the appropriate clutch apply or slip checks for that fault. If it does not return, the fault is not present at that time and may be intermittent.

Verification and takeaway

Keep the verification separate from the circuit testing. After the directed repair is completed, verify the repair and confirm P0720 stays gone. The key on this one is not to jump straight to the sensor. Work the OSS VREF and signal return circuits in order, prove the external harness, prove the internal harness, then make the repair the test path points to. For more diagnostic training, visit stepdiagnostics.com.