Honda PGM-FI Code 21: O2 Sensor — Causes & Fix
On Honda motorcycles and scooters, PGM-FI code 21 means the ECM has detected a fault in the O2 (oxygen) sensor circuit. The O2 sensor lets the ECM trim the mixture for clean, efficient running (closed-loop).
- Applies to
- Honda motorcycles
- Est. repair cost
- $25 – $150
- Time to fix
- 30–90 minutes
What it means
Common causes are a loose/corroded O2 connector, an exhaust leak upstream of the sensor, damaged wiring, or an aged/failed O2 sensor. This is a generic Honda PGM-FI (Programmed Fuel Injection) code, so it applies across Honda FI models — scooters and underbones like the Click, Vario, BeAT, PCX, ADV, and Wave, as well as CB/CBR motorcycles. Honda shows it by blinking the FI/MIL warning light: a long blink counts as ten and a short blink counts as one (e.g. one long + two short = code 12). Clearing the code usually needs Honda’s service-check reset procedure (jumping the DLC service connector, or a scan tool) after the fault is fixed; simply resetting without repair will let the light return.
Symptoms
- FI/MIL light on with code 21
- Slightly worse fuel economy
- Mild running roughness at light throttle
- May fail an emissions check
How to fix it
- 1
Inspect the O2 connector and exhaust
1. Find the O2 sensor in the exhaust header. 2. Reseat the connector and check for melted or corroded pins. 3. Look for exhaust leaks before the sensor — they skew its reading.
- 2
Test and replace the sensor
1. Check the sensor and heater circuit against spec. 2. Replace an aged or dead sensor, then clear the code.
Frequently asked questions
Can I ride with code 21?
Usually yes for a while — the ECM falls back to a preset map. But fuel economy suffers, so replace the sensor when you can.
Last updated July 15, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.