The trouble with hot engine start has come back    But no error codes or anything. So I started to read about how to test O2 sensors and found some interesting videos.

There are two fairly crude methods to test operation of O2 sensors without having test computers - one is when the sensor is removed and the other with the sensor in-place.

Both methods require the sensor cable to be disconnected and external "12V" is supplied to the heater (two white wires). A DC voltmeter is used to monitor voltage generated by the sensor element itself (the other two wires). The voltage should change when the amount of oxygen around the sensor changes. The sensor should be hot and its voltage stabilised prior to starting the test (takes about 2-3 min)

Update: Just reading about Narrowband vs Wideband types of sensors and wondering if the above methods are applicable to wideband sensors or not?


Method 1: Sensor has been taken out from the exhaust. Use a burning candle in a jar covered with a lid. Wait until the candle burns out all oxygen in the jar and can no longer burn, then quickly put the sensor in the jar - the voltage should jump up due to the lack of oxygen in the jar. Take the sensor out of the jar and the voltage should drop back to the initial value in a few seconds.

Method 2:
When testing the sensor in-situ just twist the throttle and see if the sensor reacts with a change of its output voltage.

My test results were:
- Method 1: the sensor was generating about 0.95V and this output has not changed at all when the sensor was inserted in the jar.
- Method 2: the sensor was generating less voltage (due to heat dissipation, perhaps..) but still no reaction to throttle movements.

(Observation: when the "12V" is disconnected from the heater the sensor voltage slowly goes back to zero in less than a minute. The DC through the heater element was about 0.7A so the resistance of the heater element was about 12.5V / 0.7A = 18 Ohm approx.)

The sensor is fairly new (12K miles on the odo) and it did not look dirty. I sprayed carbs cleaner into the sensor's little holes and also left it to soak in the carbs cleaner overnight. Dried and re-tested in the morning and there was no difference whatsoever. Looks like it is a faulty sensor after all.

I fitted the O2 sensor eliminator plug as a test (which is just a 320 Ohm resistor simulating the sensor heater and an open circuit for the sensor element) and I saw no difference in engine operation, idle or when riding. Not sure what to think about the 320 Ohm resistor compared to the 18 Ohm resistance of the real heater.

The behaviour is now quite consistent: the engine starts almost instantly when cold. And it almost never starts straight away when hot, but usually requires one or two "dry" start attempts (with throttle fully open and fuel supply cut) before starting normally again.

I read somewhere that the ECU would ignore the O2 sensor readings during the engine start and would initially run in the open loop mode when idling until the engine gets hot. But what if the engine is already hot? Would the faulty O2 sensor (or the lack of it) make the mixture too rich and cause engine flooding?

And another observation: I read that the heater should heat the sensor up to 600F (300C) or more for narrowband or 1000F (500C) or more for wideband types. Mine did not feel that hot when heated because I could touch the sensor body near the cable end with my bare hand.

Hello, Mike from South Cheshire. I have a Triumph Scrambler 900, looking to add a CBF1000 as I have fond memories of my old CB750FB bike back in the day !  I know that doesn't make much sense but there you go !

Time to get down and dirty and check those codes. If I was a gambling man I'd suspect a fuel or ignition systems fault to be the root cause of a rough idle. When lifting the petrol tank to check the codes check for any inlet air leaks while you're there.

Did someone mention that this was Phild's old SC64 (VO62NZU) the one with the failed exhaust servo? If so I'm wondering... have a read here

The downside of taking on a low mileage motorcycle that's barely turned a wheel since pre-Covid times, 4 years?

Oh yes, thanks for the advice, this is great. Will definitely have one installed. Can't imagine having to take off all the fairings and lift the tank every time I need to read the codes. Cheers 🙏

It can be quite a task to locate that DLC connector on SC64 without lifting the tank. Mine was wrapped in insulation tape and it took me a while to figure out its location and how to pluck it open :)
To make it futureproof I bought a plug with a switch on a cable lead: https://www.ebay.co.uk/itm/283741177977 and run that cable up so the switch is accessible from under the rider's seat - this made future diagnostics much easier.