revs hang high on their own, C15/0118 ET code, ET ruled out by test.

[Onat]

Bike Model = : 2013 Hyosung GV250 EFI

Hi all. New rider, first bike, and I’ve spent two days on this. I’d rather lay out everything I’ve found than ask a vague question, so apologies for the length.

The bike

I bought it recently. It had been sitting unused for a while before I got it.

*The symptom

– On start, with no throttle input at all, it jumps straight to 3-4k rpm
– Over a couple of minutes it steps down to about 2.5k and stops there. It never reaches a normal idle
– At 2.5k it isn’t steady , it wanders up and down slightly, continuously
– At random times, with my hand nowhere near the throttle, the revs climb on their own and then hang there. There’s no consistent number sometimes it sits around 2.5k, sometimes 5k sometimes higher. It just gets stuck at whatever level it picks
– When it’s hanging like that, blipping the throttle doesn’t clear it. The revs rise while I twist, then settle right back to the same hung level. The engine basically ignores the input
– It has never gone to redline
– It starts and stops doing this at random. No clear trigger I can identify
– It has got worse over time. When I first got the bike from the mechaniÅŸc, it happened rarely. After 60 km became much more frequent

The code

FI light is on constantly. Blink code reads as C15 / 0118 ET sensor.

What I’ve already done

1. Checked the throttle at the grip , full twist and release feels normal, no sticking
2. Found the ET sensor on the front cylinder, pulled the connector, inspected it — pins looked clean, no green corrosion or white crust
3. Cleaned both halves with electrical contact cleaner, did several connect/disconnect cycles, reseated it. No change at all. FI light still on after several key cycles
4. Ran the engine with the ET connector completely unplugged.Behaviour was identical — same 4k start, same step down to 2.5k, same wander. This is the part I can’t get past

Why I’m doubting the ET diagnosis

Every ET sensor failure I can find online describes hard starting, stalling and misfiring, the lean side of the failure. I cannot find a single case of an ET fault causing an engine to raise its own revs like this. I understand that a high-voltage ET code should push the ECU into permanent cold-start mode and raise the idle target, so the mechanism makes sense on paper, but the real-world cases don’t match my symptoms at all.

And unplugging the sensor entirely changed nothing, which suggests the ECU was already running without it, or maybe it connects instantaneously which plays with the idle rpm? No idea.

What I found that does match

A Turkish forum thread on a GT250 describes almost exactly my symptom set “revs on its own, sometimes drops, sometimes doesn’t, sometimes normal.” That owner traced it to a worn TPS. He measured it by turning the shaft slowly and watching for a jump in the voltage sweep, found one, replaced it, and then had to sort out the adjustment. He ended up setting closed throttle to 1.12V and full throttle to 4.46V and noted that the values in the FI manual don’t apply to the EFI ECU.

The intermittent nature of a worn TPS track would explain the randomness I’m seeing far better than a straightforward dead sensor would.

Where I’m stuck

Access. The V between the cylinders is extremely tight on this bike and I can barely get a hand or a probe in there. The seat bolt is chewed out from a previous mechanic so I can’t lift the seat, which means I can’t lift the tank, which means I can’t open up the work area. I’m sorting that out separately.

What I’m asking

1. Can a C15/0118 ET code sit alongside a completely separate fault? Is it plausible I have a dead ET circuit causing the light and something else entirely causing the high idle?
2. Has anyone here actually seen an ET fault produce high idle on a GV/GT250, or is that just theory?
3. If TPS is the better suspect, do the 1.12V / 4.46V figures hold for the GV250 EFI, or should I be looking for different numbers?
4. Anything else on these bikes that raises idle on its own and comes and goes? I’ve seen the ISC solenoid mentioned but I understand that would throw its own code
5. The bike sat unused for a long time (8 years, revised by a local mechanic but I don’t trust him 100%) and the fuel pump work disturbed that area recently does either of that point anywhere specific?

I have a multimeter and I’m willing to do the work, I just don’t want to throw money at the wrong part and unregulated/uneducated mechanics here. Any pointers appreciated.

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