Which motorcycles does code 21-02 apply to?

It is documented here for the Honda CBR600RR (2003-2004). Other Honda models may use the same display code with a different meaning, so confirm against your model.

Can I still ride with code 21-02?

It depends on the fault. Code 21-02 relates to the O2 (oxygen) sensor. If the bike runs poorly or a safety-related system is affected, avoid riding until it is checked. The FI light stays on until the fault is fixed and the codes are cleared.

21-02 Strongly inferred

O2 sensor high voltage

The oxygen sensor signal read high. This code is on the 2003-2004 CBR600RR only. Standard Honda meaning.

Manufacturer: Honda
Platform: PGM-FI (Keihin, Renesas M32R)
Models: CBR600RR
Years: 2003-2004
Subsystem: O2 (oxygen) sensor
FI-light blink: 2 long, 1 short
Hex: 0x2102

Saw 21-02 as 2 long, 1 short on the FI light? Decode a blink pattern →

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Strongly inferred. Strongly inferred: the standard Honda meaning is well established, but exactly how this ECU handles it internally is inferred rather than fully confirmed.

What it means

The oxygen sensor lets the ECU fine-tune fuelling from the exhaust. On the 2003-2004 bike, a high oxygen-sensor reading sets this code and closed-loop fuel trim stops. Later CBR600RR ECUs do not use this exact code.

Conditions

Detection conditions

The ECU checks the oxygen sensor signal against its limits.

Sets the fault when

The oxygen signal reads high past the fault criteria.

Clears the fault when

Fix the underlying problem, then clear the stored codes with a diagnostic tool or the manufacturer’s reset procedure. If the fault is still present, the code returns the next time the ECU detects it.

Parts involved

Oxygen (O2) sensor and its wiring

Symptoms & likely causes

Possible symptoms

FI (check-engine) light on
Closed-loop fuel trim disabled, so fuelling may be slightly off

Likely causes

Oxygen signal shorted high or to the reference
Failed oxygen sensor
Wiring or connector fault

Diagnostic guidance

Only the 2003-2004 ECU reports this exact code. Diagnose it as an oxygen-sensor circuit fault.

How the ECU decides

A simplified view of the logic behind this code: a short C snippet with a plain-English summary. It shows the real decision the ECU makes without the low-level detail.

When the ECU confirms a fault, it saves the code and turns on the FI (check-engine) light. For a serious fault it also runs the engine on safe default values (limp-home) so you can still ride, with the light on.

C How a confirmed fault is stored

1// Once a fault is confirmed, remember it and warn the rider.
2if (fault_confirmed) {
3    store_code(code);      // kept until the codes are cleared
4    fi_light = ON;
5    if (fault_is_serious)
6        limp_home = ON;    // safe defaults so you can ride home
7}

Reverse-engineered interpretation · CBR600RR ECU reverse-engineering analysis

Related codes

Questions about code 21-02

What does Honda code 21-02 mean?: The oxygen sensor signal read high. This code is on the 2003-2004 CBR600RR only. Standard Honda meaning.
Which motorcycles does code 21-02 apply to?: It is documented here for the Honda CBR600RR (2003-2004). Other Honda models may use the same display code with a different meaning, so confirm against your model.
Can I still ride with code 21-02?: It depends on the fault. Code 21-02 relates to the O2 (oxygen) sensor. If the bike runs poorly or a safety-related system is affected, avoid riding until it is checked. The FI light stays on until the fault is fixed and the codes are cleared.
What should I check first for code 21-02?: Only the 2003-2004 ECU reports this exact code. Diagnose it as an oxygen-sensor circuit fault.

Sources

Every technical claim on this page traces to the research below. Source labels are sanitized; no local research paths are exposed.

firmware

2003-2004 Honda CBR600RR ECU firmware

Factory PGM-FI engine control firmware for the 2003-2004 CBR600RR.
analysis

Firmware fault-code table extraction

Reads the list of codes the ECU can store directly out of each firmware.
reference

HondaECU project (open-source Honda ECU tool)

Community reference mapping Honda PGM-FI codes to their meanings.
analysis

CBR600RR ECU reverse-engineering analysis

Independent analysis of how the ECU detects, stores, and reports faults.

Content version 1.0.0 · last reviewed 2026-07-14

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