Hondata

1999 Civic Si with an 07 k24a2, ep3 engine harness, older Ktuned conversion harness, V1 kpro , OEM charge harness and OEM battery in the oem location. The problem started last year. While driving the car hard it will intermittently throw a check engine light for the Daughter board, I could cycle the key and it would go away for a few sessions on track. Well now it is an ongoing problem that almost instantly comes back. If i drive the car easy it runs fine but as soon as i push it, it bucks and flashes the mil and then vtec won't engage and a few moments later the mil stays on. Ive changed the battery, alternator, engine harness, charge harness and ecu with no luck. If i unplug the primary 02 sensor it runs fine though. Usually people get this code at start up but mine comes on after driving a few minutes and then only when pushing it. My voltage at the battery and the ecu are spot on. Someone guide me to the light.

The error code is just the symptom - the key is that the o2 sensor affects the problem. I'm guessing the o2 heater current is enough to move the ECU ground up from 0V and then the ECU gets a low voltage situation. I'd think that this would also occur with VTEC as well as the o2 sensor.
What grounds do you have in the engine bay? Where is the main harness ground? If they look ok then the next step would be to measure voltage then current from battery negative to the ECU ground pin, with the engine running.

All grounds are 100% I even added a secondary ground from the valve cover stud, which is where i have the ecu grounded

And i also found out that Ktuned uses the same power source for the O2 sensor and the ecu in their conversion harness

And could it be a bad 02 sensor? I went years with no problems and then all of a sudden i have this problem

What grounds do you have in the engine bay? Where is the main harness ground attached?

One from the timing chain cover to the chassis, one from the trans case to the chassis and then the main harness ground is on the valve cover stud which I have another ground wire running from that back to the chassis

Are you running a flex fuel sensor?

I am not

K-raig24 wrote: Thu Jul 06, 2017 8:33 am And could it be a bad 02 sensor? I went years with no problems and then all of a sudden i have this problem

It is possible if the o2 sensor heater has shorted out.

So it mist have been a coincident that it didnt throw the code with the 02 sensor unplugged. It just threw the code with it unplugged

Ok, the next step is to measure voltage then current from battery negative to the ECU ground pin, with the engine running.

Is this possible to disable checking for this error?
Or at least disable rev limiting to 4k when that error occurs?

In my rally car I have smaller battery, and 2.5m long ground wire (thru emergency power cut-off switch in cabin).
That battery is enough for start an engine without any problems, but probably is not enough to give over 10V all the time when engine starting is working.

On stock ECU everything was fine, but today I run my car first time with Hondata KPro, and constantly got that error.
Biggest problem with that is revlimit to 4k.

I second a function to disable that error, but I doubt is it possible as the error starts even before starting the engine.... have you tried to just put the key into ignition and wait (without starting the car). If you're like me, the error comes up during the electronics check, even when the battery voltage is above 12v. You can check this by looking at the led light on the ECU, it will change to red flashing from green (during electronics check, before starting the motor). I'm still dealing with a newborn baby, so haven't had time to rip apart my wiring and no one is willing to lend me a car to throw the ecu into, as most people are afraid that it will damage the car or their electronics.

Sorry, the error cannot be disabled. You will have to look at the reason why the battery voltage is dropping.