P0720 on 2006-2011 Honda Civic Si: Output Speed Sensor Causes and Fixes

What's Unique About the 2006-2011 Honda Civic Si

The 2006-2011 Civic Si (8th generation) was sold exclusively with a 6-speed manual transmission. While P0720 is a standard code, many generic guides will mention symptoms like erratic automatic shifting or a blinking 'D' (Drive) light, which do not apply to the Si model. For the Civic Si, the main effects are an inoperative speedometer and potential issues with engine performance, as the ECU loses a critical data point for its calculations. The failure of this specific sensor is a very common and well-documented issue on this platform and its Canadian-market twin, the Acura CSX Type-S.

Symptoms You May Notice

• Check Engine Light is on
• Speedometer is inoperative, reads 0 mph, or behaves erratically
• Cruise control does not work
• Possible engine hesitation or stumbling
• VTEC engagement may be erratic or disabled
• Engine may feel sluggish or have reduced power

Most Likely Causes

1. Faulty Output Shaft Speed Sensor 🔴 High Probability The sensor is a known common failure item on this platform due to internal electrical failure over time and exposure to heat cycles from the transmission.
   How to confirm: Use a scan tool to monitor 'Output Shaft Speed' live data while driving. If the reading stays at 0 mph while the vehicle is moving, the sensor or its circuit has failed. Resistance testing the sensor itself can also confirm failure; an infinite resistance reading means it's internally open. A good sensor should have a specific resistance value which can be found in the factory service manual.
   Typical fix: Replace the output shaft speed sensor.
   Est. part cost: $20-$70
2. Damaged Wiring or Corroded Connector 🟡 Medium Probability The wiring harness is located on top of the transmission and can be subject to heat, vibration, and environmental exposure, potentially causing wires to break or connectors to become loose or corroded over time.
   How to confirm: Visually inspect the wiring harness leading to the sensor for any signs of cracking, fraying, or melting. Disconnect the sensor and check the connector terminals for corrosion or damage. A user on CivicForums.com reported finding their wires completely severed 🎬 Watch: Diagnosing the P0720 sensor circuit, likely by a rodent.
   Typical fix: Repair the damaged section of wire or clean/replace the connector pigtail. Ensure any repairs are properly soldered and heat-shrunk to prevent future corrosion.
   Est. part cost: $5-$30

Rare But Worth Checking

• Rodent Damage to Wiring: Rodents can be attracted to the soy-based wiring insulation used in many modern cars, including Hondas. A user on a popular Civic forum found their sensor wires were completely cut, causing a P0720 code.
• Faulty Powertrain Control Module (PCM): This is very rare. The PCM should only be considered as the cause after the sensor and wiring have been thoroughly tested and confirmed to be good.
• Metal Debris on Sensor Tip: The output speed sensor is magnetic. In transmissions with high mileage or internal wear, fine metallic particles can accumulate on the sensor's tip, disrupting its ability to generate a clean signal. When replacing the sensor, it's wise to inspect the old one for metal shavings, as this could indicate a need for a transmission fluid change or a more serious internal transmission problem.

Diagnosis Steps

1. Connect an OBD-II scanner and confirm the presence of the P0720 code. Check for any other related codes, like P0500.
2. Use the scanner's live data function to monitor the 'Output Shaft Speed' or 'Countershaft Speed' PID (Parameter ID). On a Honda-specific scanner (HDS), this is listed as 'C SHAFT SPD'.
3. Safely raise the vehicle on jack stands or drive it while a passenger monitors the live data. If the vehicle is moving but the speed reads 0, a problem with the sensor or circuit is confirmed.
4. Turn off the vehicle and locate the output shaft speed sensor on the top/rear of the transmission housing. Access requires removing the air intake assembly. The sensor is held in by a single 10mm bolt.
5. Inspect the sensor's connector and wiring harness for any visible damage, corrosion, or loose connections. Pay close attention to the area where the harness might rub against other components.
6. If wiring appears okay, disconnect the sensor. Measure the resistance between the two terminals on the sensor. A good sensor should have a resistance between 400-600 Ohms. A failed one will often show infinite resistance (open circuit).
🎬 Watch: How to bench test a Honda speed sensor
7. If the sensor tests bad, replace it. If the sensor tests good, the problem lies in the wiring harness or the PCM. Check for voltage at the connector with the key on, engine off. According to the service manual, you should have approximately 5V at terminal 1 (to ground) and terminal 2 (to ground). A test between terminals 1 and 3 should also show about 5V. Lack of voltage points to a wiring issue between the ECM and the sensor.

Parts You'll Likely Need

• Output Shaft (Countershaft) Speed Sensor (OEM #28810-PWR-013) — This is the most common failure point for code P0720 on this specific vehicle, confirmed across multiple forums and repair databases.
  Trusted brands: Honda (Genuine OEM), NTK, Denso, Standard Motor Products (SMP), Walker Products
  OEM price range: $50-$70
  Aftermarket price range: $20-$50

Related Codes That Often Appear With This One

• P0500 — P0500 is a generic code for 'Vehicle Speed Sensor Malfunction'. Since the output shaft speed sensor data is used to determine vehicle speed, a fault in the P0720 circuit can also trigger P0500.

Technical Service Bulletins (TSBs) & Recalls

• Honda TSB 08-020: While not for P0720, this is a critical TSB for the 2006-2008 Civic Si 6-speed manual transmission. It addresses grinding, popping out of 3rd gear, or hard shifting into 3rd due to a faulty 3rd gear synchronizer or 3-4 shift sleeve.

Platform-Specific Known Issues

• Owner Experience: Speedometer Died, P0720 Thrown: A user on 8thcivic.com described their 2007 Si's speedometer suddenly dropping to zero while driving, followed by the Check Engine Light illuminating with code P0720. They replaced the countershaft speed sensor (part #28810-PWR-013) located on top of the transmission, and the issue was completely resolved. The repair was noted as being simple, requiring only the removal of the intake to gain access.

Mechanic-Grade Diagnostic Values

• Output Shaft (Countershaft) Speed Sensor Resistance — expected: 400 - 600 Ohms. Failure: Infinite resistance (open circuit) or a reading far outside the specified range.
• Sensor Connector Voltage (Key On, Engine Off) — expected: Approximately 5.0V between terminal 1 and ground, and approximately 5.0V between terminal 2 and ground.. Failure: No voltage or significantly low voltage indicates an open or short in the wiring to the ECM.
• Sensor Connector Voltage (Key On, Engine Off) — expected: Approximately 5.0V between terminal 1 and terminal 3.. Failure: No voltage indicates an open in the ground wire (terminal 3) back to the ECM.
• Live Data on Scan Tool (while driving) — expected: PID 'C SHAFT SPD' should show a speed reading that increases as the car moves.. Failure: Reading remains at 0 while the vehicle is in motion.

Scan Tool Commands That Help

• Honda Diagnostic System (HDS): DATA LIST -> C SHAFT SPD — This is the primary function used to confirm the fault. It allows the technician to see the speed value the PCM is receiving from the sensor in real-time. If the value is 0 while the car is moving, it confirms the sensor or its circuit is not sending a signal.
• Honda Diagnostic System (HDS): Reset ECM — After replacing the sensor or repairing the wiring, the ECM should be reset to clear the stored fault code and any learned values. This ensures the system re-evaluates the new, correct signal.

Wiring & Ground Locations

• Output Shaft (Countershaft) Speed Sensor 3P Connector — On top of the transmission housing, towards the rear of the engine bay. It is located underneath the air filter box assembly.. This is the primary point for testing the sensor and the circuit. All voltage and resistance checks are performed at this connector.
• ECM Connector B and C — Located in the engine bay, on the passenger side, near the firewall.. The sensor wires terminate here. Specifically, the service manual points to pins B18 and B33 for circuit checks if voltage is missing at the sensor connector.
• G101 (Engine Ground) — Typically located on the thermostat housing, grounding the engine harness to the engine block.. A poor main engine ground can cause a host of intermittent and strange electrical issues, including faulty sensor readings. While not a direct cause of P0720, ensuring G101 is clean and tight is a crucial step in diagnosing any electrical fault on a Honda.

Real Owner Repair Stories

• CivicForums.com user (Unknown year Honda Civic) — Check Engine Light with code P0720.
  ❌ Tried (didn't work) Initial repair attempt with twisted and taped wires was not successful.
  ✅ What actually fixed it The root cause was found to be completely severed sensor wires, likely from a rodent. The final fix required a proper solder and heat-shrink repair of the damaged wiring harness to restore a solid connection.
• Honda Owner Report (NHTSA ODI #10339099) — While reported on a different Honda model, an owner noted that diagnostic code P0720 can sometimes be an early sign of broader transmission failure, even when the dealership attempts to resolve it by simply replacing the sensor.

OEM Part Supersession History

• 28810-PWA-003 → 28810-PWR-013 — The 28810-PWR-013 is an updated version with improved shielding against electromagnetic interference and a more durable internal coil winding.
  Heads up: While the older -PWA-003 part from 2003-2005 Civics may physically fit, the -PWR-013 is the correct, more robust part for the 2006-2011 generation.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• 3rd Gear Grind / Pop-Out 🔴 High — Very common, especially on 2006-2008 models. Can occur at any mileage. Symptoms are often worse when the transmission is cold. (Ref: Honda TSB 08-020)
• Paint and Clear Coat Failure 🟠 Medium — Extremely common, particularly on dark colors like Nighthawk Black Pearl, Fiji Blue Pearl, and Galaxy Gray Metallic. The clear coat begins to oxidize, cloud, or peel, typically on the roof, hood, and trunk after 5-10 years. (Ref: Honda Service Bulletin 14-034 (Warranty Extension, now expired))
• Cracked Engine Block (K20Z3) 🔴 High — Less common than the R18 engine's issue, but still a documented failure mode for the K20Z3, often due to casting defects. It causes coolant to leak from the back of the block, leading to overheating. Honda had a 10-year warranty extension which has since expired. (Ref: Honda TSB 10-048 (Expired Warranty Extension))
• Cracked Sun Visors 🟡 Low — A very common annoyance where the plastic housing for the sun visor cracks at the pivot point, causing it to droop.
• Worn Upper Strut Mount Damper Caps 🟡 Low — A common source of rattling from the front of the vehicle. (Ref: Honda TSB 20-036)

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: Buying a used sensor is generally not recommended. This part is a common failure item due to internal electronics breaking down over time. The low cost of a new, quality aftermarket part (around $20-$50) makes it a poor value proposition to risk installing a used sensor with unknown remaining life.

Donor-vehicle mileage cap: roughly under 50000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• If buying used is unavoidable, inspect the connector for any cracks or broken locking tabs.
• Check the plastic housing for any signs of heat stress or melting.
• Ensure the O-ring is present and not flattened or cracked, though it should be replaced regardless.
• Test the resistance with a multimeter before purchase; it must be within the 400-600 Ohm range.

OEM-only on this vehicle (don't cheap out):

• For this repair, OEM is not strictly necessary, but it is the safest bet for guaranteed compatibility and longevity.

Aftermarket brands forum-validated for this vehicle:

• NTK
• Denso
• Standard Motor Products (SMP)
• Walker Products
• Delphi

Brands owners have reported issues with on this vehicle:

• Avoid unbranded, no-name sensors from online marketplaces, as their internal electronics and resistance values can be inconsistent, leading to a quick return of the P0720 code.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0500 — A companion code often checked alongside P0720, as both relate to vehicle speed sensor circuit malfunctions.
• P1297 — Has been observed appearing alongside P0720 in cases involving broader electrical failures or ELD (Electric Load Detector) issues.
• TSB 08-020 — While not a diagnostic code, this TSB is frequently discussed in the same context as Civic Si transmission issues, specifically regarding 3rd gear synchronizer failure.
• TSB 10-048 — Related to the K20Z3 engine platform found in the Civic Si, addressing potential cracked engine blocks and coolant leaks.

Frequently Asked Questions