F1 Light, code 24,25 clarification

ab-buff

ab-buff

Joined
Mar 10, 2025
Messages
19
Age
72
Location
Tracy California
Bike
2004 ST1300
Hi everybody
First off merry Christmas to everybody.
I have been reading a lot about this code. I am going to dig into my bike over the next couple of weeks. I need to replace some hoses, thermostat change of fluid, etc. so I’m going to go fairly deep. I keep reading that everybody says the F1 code 24 and 25 is a knock sensor. If you look on the chart in the manual it’s an O2 sensor that’s a different animal. I’m kinda confused why everybody says it’s the knock sensor. Maybe that’s why when they change it or do something with it. It’s not fixed then they change the ECU? Anyway, the O2 sensor is another part that’s not available but you can pick them up used if you need to. Can anybody shed some light on this? I guess my question is is why does everybody say knock sensor?
 

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Well, if I had to turn the page in the book, I would’ve figured it out. Kind of feel like a dummy. See attached.
 

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One more note on the knock sensor. They are available brand new from Partzilla. Going to replace both of them. Along with the wire harness.
 
It was 25 or 26 for some reason I can’t seem to bring it back up. I have managed to get the F1 light on again a couple of times, but still unable to read the code.
 
ab-buff said:
It was 25 or 26 for some reason I can’t seem to bring it back up. I have managed to get the F1 light on again a couple of times, but still unable to read the code.
Click to expand...
If it is the problem that I suspect it is, the error light will automatically reset after stopping the engine. One of the tests that can trigger one of the knock sensor codes can be done in your garage with the bike on its center stand.

After placing the bike on its center stand, start the bike and let it idle. Then, slowly increase the throttle until the engine reaches 4,200 rpms. Continue to hold the engine at 4,200 rpm's for a minute or two. This action should trigger the knock sensor code and you can return the engine to idle speed. Instead of shutting off the engine, let it continue to idle and count the flashes. You may want to let the series of flashes repeat one or two times to make sure you've counted correctly. Then, turn off the engine with the stop switch and turn the key to its off position. In a moment or two, turn the key back to its on position and flip the stop switch to run. The FI light should come on briefly and then turn off and remain off.

If raising the rpms triggers the FI light to flash a code and resets by using the stop button, then you will need to follow the diagnostic process set forth in the Service Manual. In my case, my knock sensors checked out fine, and I had full circuit continuity between the sensors to the PGM-FI unit (i.e ECM). If you find yourself in the same position, then you can safely conclude your ECM has gone bad.

As you may already know, the ECM for 2003-2007 models sold in the US are no longer made by Honda. However, the Police model ECU will work as an acceptable substitute. Unfortunately, it, too, is no longer made by Honda. However, I found a new Police ECU from a Japanese distributor and had it imported to the US. That said, a new ECM is not cheap. You might find a second-hand ECM that is not throwing the knock sensor code, but they are a rare find. In my case, I elected to buy a new one because I had just rebuilt the brake calipers, replaced the SMC and rear caliper bracket, new final drive bearings, front wheel bearing, as well as new steering stem bearings. Having invested a good deal of time on my '05 at that point, I felt buying a new ECM was cheaper than buying a new bike. Of course, YMMV.

Hopefully, you won't need to replace your ECM. In that case, you'll likely need to make a wiring repair, as the odds that it is the knock sensors are about the same odds as winning the Mega Millions jackpot. I'll keep my fingers crossed for you.

Chris
 
Buckeyechris said:
This action should trigger the knock sensor code and you can return the engine to idle speed. Instead of shutting off the engine, let it continue to idle and count the flashes. You may want to let the series of flashes repeat one or two times to make sure you've counted correctly. Then, turn off the engine with the stop switch and turn the key to its off position. In a moment or two, turn the key back to its on position and flip the stop switch to run. The FI light should come on briefly and then turn off and remain off.

If raising the rpms triggers the FI light to flash a code and resets by using the stop button, then you will need to follow the diagnostic process set forth in the Service Manual.
Click to expand...
I know in my case, triggering either knock sensor code with RPM above around 4k for more than about 20" causes the FI light to come on steady, and it doesn't flash as described above, but it does reset with an engine cut off switch cycle (ignition switch cycle not required) and can be 'reset' on the fly.

To 'read' the code I'm pretty sure you have to jumper the service check connector per the manual; I don't know of any code that auto displays when triggered, but maybe I just haven't experienced it yet?
 
Gus1300 said:
I know in my case, triggering either knock sensor code with RPM above around 4k for more than about 20" causes the FI light to come on steady, and it doesn't flash as described above, but it does reset with an engine cut off switch cycle (ignition switch cycle not required) and can be 'reset' on the fly.

To 'read' the code I'm pretty sure you have to jumper the service check connector per the manual; I don't know of any code that auto displays when triggered, but maybe I just haven't experienced it yet?
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Gus,

Thank you for the reminder. It's been a few years since I had this problem, and my memory was a little hazy.

As Gus said, it should take seconds, not minutes, to trigger the error code. He is also correct that the FI light will not flash while the engine is running. Instead, it will begin flashing once the engine is off by using the stop switch. The FI light will continue to flash the code sequence until the key is turned to the off position.

In my situation, I did not need to use the jumper to reset the ECM. The reset would automatically reset once the key was turned to the off position.

My apology for the mistake.

Chris
 
Well, I finally retrieve the code this morning put the bike on the center stand started it up. Let it warm up got to two bars on the temperature gauge I turned it off, put the side stand down put the jumper in turn the key on and got nothing. So I Started it up. I had taken the jumper out the side stand was down. I revved it up above 4100 to 5,000 a few times kind of kept it at that rpm and then the F1 light came on and flashed two long blinks and five short ones so code 25. I watched it a couple of times to make sure that was it. thank you guys for helping me retrieve that. So I’m going to dig into this thing in the next few days. Just have to finish a couple other small projects around here. I will be replacing the knock sensors with brand new ones and the a new lower wire harness. There are some other things I’m going to do change, some water hoses, thermostat. I think there’s a five way vacuum hose that needs to be cleaned?? a number of items. I’d like to take care of all of that while I’m in there. Thank you guys for the help. I’ll be posting my progress or questions as I get into this thing.
 
ab-buff said:
Well, I finally retrieve the code this morning put the bike on the center stand started it up. Let it warm up got to two bars on the temperature gauge I turned it off, put the side stand down put the jumper in turn the key on and got nothing. So I Started it up. I had taken the jumper out the side stand was down. I revved it up above 4100 to 5,000 a few times kind of kept it at that rpm and then the F1 light came on and flashed two long blinks and five short ones so code 25. I watched it a couple of times to make sure that was it. thank you guys for helping me retrieve that. So I’m going to dig into this thing in the next few days. Just have to finish a couple other small projects around here. I will be replacing the knock sensors with brand new ones and the a new lower wire harness. There are some other things I’m going to do change, some water hoses, thermostat. I think there’s a five way vacuum hose that needs to be cleaned?? a number of items. I’d like to take care of all of that while I’m in there. Thank you guys for the help. I’ll be posting my progress or questions as I get into this thing.
Click to expand...
Wishing you the best of luck. It's almost never a problem with the knock sensors themselves. I'd rule out other problems first, because the replace process is kinda a pain.

If you have the Honda Service Manual, my version had the color code for the wires running from the knock sensors to the ECM was incorrect. However, you can easily see the wiring color and adjust accordingly. As I may have mentioned earlier, the continuity of my wiring from the sensors to the ECM checked out. I jumped the wires from each knock sensor to the opposite side to see if it made a difference. In my case, I got the same error code, so I knew it was not the sensors.

Chris
 
You can start by swaping the left and right sensors. If the 25 sensor is really faulty, the code should switch to 26 once installed on the other side.
 
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