My Gixxer SF250 issues: Found reasons after O2 sensor self diagnosis

According to the manual, the voltage reading should be between 0-1v at idle.

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I have an idle hunting issue when the bike is warmed up, so I decided to diagnose my O2 sensor. And the diagnosis was very interesting. I might’ve found the reason for my engine’s poor running and vibes once it was warmed up

O2 Sensor diagnosis:

The O2 sensor is placed on the exhaust right before the cat. There is only one. You can find the connector that goes to the ECU right behind the bottom side panel that covers the frame, among other wires. Unclipped the connector and there were 4 terminals.

This is the layout of the O2 sensor connector I got from a Spanish Gixxer 250 service manual. Google translate was of great help.

First two terminals give you the voltage output from the sensor. This is what ECU reads and uses to adapt the AFR in closed loop fuelling. The bottom two terminals are for the heater element inside the sensor. You can also spot them by looking for two similar colored cables coming to the sensor among the 4 that is on there. It was black colored on the Gixxer.

Voltage readings:

According to the manual, the voltage reading should be between 0-1v at idle. And it should fluctuate. Voltage reading I got was 0.00V initially and it increased to 0.02V a minute later but it stayed there without any change Even when I revved the engine to 4-5k rpms, the voltage reading was not changing at all.

Heater element resistance readings:

The manual says the resistance between bottom two terminals should be between 5.49-6.91 ohms

The reading I got after multiple checks was 9.0 ohm. Again not in the range

Observations after disconnecting the O2 sensor:

Disconnecting the O2 sensor basically puts the bike into open loop fuelling mode always. Open loop fuelling works off the default fuel maps coded into the ECU. It doesn’t read the o2 sensor values and adapt the fuelling to AFR values that is required for the emission norms and economy. It basically runs a richer map.

It also lights up an engine malfunction light along with an FI error on the console. Because the ECU looks for the sensor and since it cannot read the signal anymore, it throws a check engine light. This can also happen if your sensor has completely failed. From what I’ve understood, if the heating element has not failed, it won’t show a CEL. O2 sensor delete and dongles that trick the ECU into thinking the sensor is still there are popular among aftermarket tuning crowd.

So my bike is now stuck in open loop fuelling after disconnecting the O2 sensor.

  • The bike now runs extremely well. Pulls all the way to 9k with no flatspots even when the bike is hot.
  • The fuelling woes and hesitation is completely gone. Before, if I opened the throttle slightly from a closed position, the engine used to bog for a few milliseconds. This was an issue when you pick the throttle up after an upshift. Made smooth progress difficult.
  • Significant reduction in hand numbing vibes at cruising speeds. I’ll run it for a couple of 100 kilometers before I confirm this but there is certainly a huge difference.
  • Low end fuelling got much nicer and I could finally understand what the reviewers were praising.
  • Smooth rev-matched downshifts. The blips felt lethargic before.
  • The unstable idle is gone and bikes idles at a stable RPM when it is hot.

My conclusion from the diagnosis and experiment is that my O2 sensor is shot and it is not giving the ECU the right values and is causing the fuelling in closed loop mode to go haywire. I would like to know the inputs of you guys on this. I was not expecting such an early failure for the sensor. The bike is just 2 years and 17k old. I’ll ride the bike with the sensor disconnected for a while to confirm my findings.

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