Fix P0720: Mechanic’s solutions that WORKED

The code P0720: Output Speed Sensor Circuit Malfunction indicates an anomaly in the vehicle speed signal, primarily emanating from issues with the Transmission Control Module (TCM) or speed sensors.

P0720 quick overview

Meaning	P0720: Output Speed Sensor Circuit Malfunction
Is it serious?	Yes, it can affect the vehicle’s performance, transmission operation, and safety.
Possible causes	– Faulty Transmission Control Module (TCM) – Grounding issues in the related circuit – Vehicle speed sensor signal inconsistencies – Damaged or malfunctioning revolution sensor – Oil leaks or contamination in the crankshaft position sensor connector
How to diagnose?	Inspect the TCM: Begin by examining the Transmission Control Module for any abnormalities, damage, or signs of malfunction. Check for grounding issues: Examine all grounding connections related to the relevant circuit to ensure they are secure and undamaged. Review the vehicle speed sensor signal: Verify that the fault code aligns with the vehicle speed sensor and not an RPM signal. Examine the crank sensor: Look for signs of damage or oil contamination that might impede its function. Inspect the crankshaft position sensor: Particularly look for oil in its connector as this could affect the signal transmission. If found, clean or replace as necessary.

How to fix P0720 – learn from mechanics

I looked into P0720 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Infiniti

Vehicle Information

• 2003 Infiniti I35 3.5L

Issue + Repair History

• Transmission slips intermittently on highway and sometimes during 1-2 shift.
• Pan inspection showed no metal, fluid in good condition.
• Scan data: Turbine speed sometimes reads 0, ASCD-A/T speed sometimes reads 0.
• Speedometer once showed 0 mph.
• Only fault code p1574 returned after clearing codes.
• Previous repair history: Transmission service due to customer feeling transmission was acting strange.

Other OBD2 Codes + Test Results

• p0720: Output Speed Sensor
• p1574: ASCD VHL speed sensor
• p0011: V V T

Suggestions from the Community

1. Replace the speed sensor on the rear; might be a common issue with mismatched connectors.
2. A PO72O might be related to a faulty fluid temperature sensor.

Final Fix

• Replaced the revolution sensor; problem never recurred.

Case #2: Subaru

Vehicle Information

• 1998 Subaru Legacy Outback 2.5L

Issue + Repair History

• Installed a new motor into the vehicle.
• Fault code p0720, temp light flashing 16 times observed.
• Transmission seems to stay in first gear too long, irrespective of temperature, after initial startup.
• Issue resolves itself after the first shift, but seems like the AWD is active when it shouldn’t be.
• Speedometer works fine, but there’s confusion about the source of its reading (front or rear of the transmission).
• After inserting a fuse in the fwd fuse holder on the right strut tower, binding on turns stopped and no flashing of the ATF temp light was observed.
• Previous repair history: Motor replacement.

Other OBD2 Codes + Test Results

• p0720

Suggestions from the Community

1. Possible pinching of a wire or cracking of a solenoid on the valve body due to using a jack under the trans pan while changing the motor.
2. Check the wiring for VSS-1 & VSS-2 and scan values for these sensors in trans data. Consider replacing both if the wiring appears intact.
3. Front speed sensor 2 might be responsible for the speedometer reading, and the PO720 possibly refers to the rear speed sensor 1 of the transmission.
4. The trans pan might have been jacked on, which can interfere with solenoids and ground wire.
5. Inspect wiring for damage at the lower pan and trans area which might have been caused by jacking.
6. Check the transmission connection at the bellhousing area, as the rubber seal inside the connector may swell and prevent proper seating.

Final Fix

• The problem might have been caused during the motor installation, but no confirmation or further action was detailed. The technician appreciated the community’s responses but didn’t specify the exact solution.

Case #3: Ford

Original Problem:

• The truck had a slipping condition.
• Initial diagnostic scans showed P0500 and P0720 codes.
• A remanufactured transmission was installed, but the P0720 persisted.
• After many interventions (PCM replacement, another reman transmission, etc.), the truck still exhibited issues.
• The truck showed speed registration at a stop, which is abnormal.
• A variety of tests were conducted, including ones on the alternator, battery, and various wiring traces, but no conclusive issues were found.
  

Potential Fixes Tried:

1. Extensive wiring and ground circuit checks were performed, and issues were rectified.
2. The Transmission Range sensor was replaced due to errors.
3. Other interventions: replaced PCM, IDM, GEM, ABS modules, tested all relevant wiring and circuits.
   

Final Findings:

1. Intermittent 5 volts of AC current were found on the injector pressure regulator circuit. The connection in connector #138 was corrected.
2. The engine cam position sensor was found to be out of specification and was replaced.
3. The digital range sensor was found to be out of spec and was replaced.
4. However, the PO720 code still continued to set during rack testing.
5. Upon testing other similar trucks, it was found that they exhibited the same issue while on the lift. This suggests a software malfunction or glitch specific to this Ford model.
6. The original cause of the PO720 code seems to be linked to the weak batteries that were connected. Once they were disconnected, the problem did not recur.
   

Responses from Other Technicians:

1. One mentioned a potential issue with a PTO and showed skepticism about Jasper transmissions.
2. Another suspected a bad PSOM (Programmable Speedometer/Odometer Module) module, which converts AC signal from the VSS to DC.
3. Another remembered a similar issue with a Ford small motorhome, where the problem was in the dashboard and was caused by the tilt wheel.
4. One believed there might be a backfeed issue and suggested checking all fuses and ensuring the IDM was not water-soaked.
5. Another clarified that the p0720 code is related to the trans output shaft speed sensor, not VSS.

Finally fixed by solving the low-voltage caused by batteries and alternators.

Case #4: Mercedes

Vehicle Information:

• 2000 Mercedes-Benz C230, 2.3L / 4 cyl / Gas / Supercharged / DOHC

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Issue + Repair History:

• Vehicle won’t shift out of 1st gear.
• Shifter won’t move from neutral to park without shutting the car off.
• No backup lights.
• Code P0720 appears and returns even after being cleared.
• Car sat in another shop for over 7 weeks.

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Other OBD2 Codes + Test Results:

• Only code mentioned: P0720

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Suggestions from Community:

• Check transmission neutral safety switch.
• Consider the conductor plate on the valve body or the transmission shift module.
• Potential need to code a new shifter.
• Shifter might be “plug and play”, no additional programming required.
• Possible repair kit available for the shifter instead of a full replacement.

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Final Fix:

• Replacing the transmission control module resolved the issue. No return of the P0720 code and the transmission began shifting correctly.

Case #5: Nissan

Vehicle Information:

• 2006 Nissan Altima, 2.5L

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Issue + Repair History:

• MIL lamp on.
• RPM signal from PCM to TCM has incorrect voltage.
• At times, vehicle shifts late, but mostly operates without noticeable problems.
• Tachometer displays correct RPM, but it’s sourced from the same wire as the TCM which reads incorrect voltage.
• Original code mentioned was P0720, later corrected to P0725.

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Other OBD2 Codes + Test Results:

• P0725: Engine speed signal.

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Suggestions from Community:

• Check for a grounding issue which could elevate voltage.
• Code might refer to the vehicle speed sensor signal rather than RPM signal.
• Check the crank sensor for oil as it can cause problems.
• Consider the crankshaft position sensor, especially if oil is found in its connector.

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Final Fix:

• After multiple checks, the TCM was identified as problematic.
• However, the customer declined repairs.
• A crank sensor was installed but no change was observed.