Article [13] ST1300 - FI Codes (MIL) Malfunction Indicator Lamp

@Igofar White Courtesy Phone consult: (Thank you Larry for your time and advice!)
Spark plugs tight, ensure no counterfeit!!
inspect coils; including grounds
No exhaust wrap! (PRO; insulates engine from exhaust pipe radiant heat; heat goes out tail pipe. CONS: causes condensation corrosion)
inspect knock sensor sub-harness
Turn off via kill switch, then ignition
3 bars idle down before ride
Replace Battery (2018)
Throttle Body sync. DONE
Check all wiring; especially grounds

I cleaned and dialectic greased coil mounts and low voltage coil connectors
I checked spark plugs, I overlooked during my general overhaul last year.

Manual calls for:
Stock: NGK CR7EH-9
(.031-.035)
Optional: NGK CR8EH-9
8-12k miles interval

Plugs installed by PO NGK CR9EH-9 (cooler for sustained high speed)
12K+ miles

I ordered set of NGK CR7EH-9

Spark Plug wires may be original (70k+ miles; 20 years); some sources say OEMs are inferior to what's now available.

Recommendations/sources welcome for replacement plug wires

UPDATE; I swapped knock sensors. Code 25 stays with harness?
 
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eyrepb said:
@Igofar White Courtesy Phone consult: (Thank you Larry for your time and advice!)
Spark plugs tight, ensure no counterfeit!!
inspect coils; including grounds
No exhaust wrap! (PRO; insulates engine from exhaust pipe radiant heat; heat goes out tail pipe. CONS: causes condensation corrosion)
inspect knock sensor sub-harness
Turn off via kill switch, then ignition
3 bars idle down before ride
Replace Battery (2018)
Throttle Body sync. DONE
Check all wiring; especially grounds

I cleaned and dialectic greased coil mounts and low voltage coil connectors
I checked spark plugs, I overlooked during my general overhaul last year.

Manual calls for:
Stock: NGK CR7EH-9
(.031-.035)
Optional: NGK CR8EH-9
8-12k miles interval

Plugs installed by PO NGK CR9EH-9 (cooler for sustained high speed)
12K+ miles

I ordered set of NGK CR7EH-9

Spark Plug wires may be original (70k+ miles; 20 years); some sources say OEMs are inferior to what's now available.

Recommendations/sources welcome for replacement plug wires

UPDATE; I swapped knock sensors. Code 25 stays with harness?
Click to expand...
Brad,

I am sorry to hear that you're still getting error code 25 after swapping sensors. Have you gone through the trouble shooting process outlined in the Honda Service manual?

Unfortunately, this error code is usually never the result of a faulty knock sensor. It usually means one of two things, you have a broken or damaged wire between the left knock sensor and the ECM or a part in the ECM has gone bad. In my case, it was latter. Ultimately, I elected to purchase a new ECM. However, I should note that Honda no longer manufactures this ECM for pre '08 STs. I did, however, find a supplier in Japan that had a police ECM in stock. That ECM can be used in place of the civilian model. But it does limit the top speed of the ST to 120 miles per hour. This was not an issue for me, but concerns about the limiter have been expressed on the forum.

While I wasn't comfortable doing this, many have continued to ride their STs without replacing the ECM. Although it will lessen fuel economy, most swear they don't notice much of a difference. @Igofar believes operating the bike with the error code is damaging. It's his observation that STs with damaged ECMs run hotter, which can damage and melt plastics. YMMV.

Good luck on your diagnostics.

Chris
 
Am going thru manual troubleshooting. 5-47 states:

Screenshot 2026-03-26 at 8.40.59 PM.png
Don't we want continuity from the knock sensor connector 1P to the ECM connector? If so (YES), wouldn't that be a closed circuit? Am I misunderstanding something?






Screenshot 2026-03-26 at 8.40.59 PM.png
 
eyrepb said:
Am going thru manual troubleshooting. 5-47 states:

Screenshot 2026-03-26 at 8.40.59 PM.png
Don't we want continuity from the knock sensor connector 1P to the ECM connector? If so (YES), wouldn't that be a closed circuit? Am I misunderstanding something?






Screenshot 2026-03-26 at 8.40.59 PM.png
Click to expand...
Brad,

You are correct. Continuity is what should be the result if the wiring is sound and intact.

This represents one of the quirks that sometimes happens with publications by Mother Honda. Something got lost in translation. Whether it is a Japanese to English issue or an Engineer to Publisher issue, I cannot be sure. It's been one of my frustrations for years. Nevertheless, the HSM is correct at least 90 percent or more, so I just go with it. If I don't think it's correct, I check with other sources. Thankfully, ST-Owners is a very supportive forum and people more knowledgeable and skilled than I have been willing to help me out. I'm just trying to give you the limited benefit of my experience. A certain level of aptitude is always helpful, and your aptitude is rightfully questioning the folks at Honda. While they're really good, they're not perfect.

Chris
 
Buckeyechris said:
Although it will lessen fuel economy
Click to expand...
Only if you continue to run with the light illuminated; IME it does not affect fuel consumption when no light is illuminated as the bike is not in reversion fuel mapping like it is when the fault triggers/light is illuminated...still seeing upper 40mps (and low 50mpg a few times too, now that I've replaced the aftermarket TwoBros mine had when I bought it with an OEM exhaust) The bike also sounds/feels different throughout the RPM band for some reason, if anyone can explain that...maybe because it's now seeing the correct backpressure through the tubes that the aftermarket didn't present? It is definitely quieter too.

I do see a significant MPG drop when I lose track and accidentally trigger the fault; once reset (quick engine cutoff OFF>ON, restart), it takes a few minutes for the throttle sensitivity to disappear and everything to be back to normal. I have not yet worked through the troubleshooting steps as my right side connector is completely melted (possible support of Igofar's claim of higher heat but having ridden a lot in FL and NV, I can't say for sure when it actually melted or relate that to when I started getting the codes) and I'm done trying new ECMs (been through three of them with same end result) until I can get to the sub-harness to replace it and see if that resolves anything. Exceeding the RPM limit/time should be less of an issue with the new McCruise (love it!) that I can just set at 3950RPM (slight red of the 4K line showing just above the needle) and forget about throttling for awhile on the highway. I also need to update my picture...over 151k now.
 
Gus1300 said:
Only if you continue to run with the light illuminated; IME it does not affect fuel consumption when no light is illuminated as the bike is not in reversion fuel mapping like it is when the fault triggers/light is illuminated...still seeing upper 40mps (and low 50mpg a few times too, now that I've replaced the aftermarket TwoBros mine had when I bought it with an OEM exhaust) The bike also sounds/feels different throughout the RPM band for some reason, if anyone can explain that...maybe because it's now seeing the correct backpressure through the tubes that the aftermarket didn't present? It is definitely quieter too.

I do see a significant MPG drop when I lose track and accidentally trigger the fault; once reset (quick engine cutoff OFF>ON, restart), it takes a few minutes for the throttle sensitivity to disappear and everything to be back to normal. I have not yet worked through the troubleshooting steps as my right side connector is completely melted (possible support of Igofar's claim of higher heat but having ridden a lot in FL and NV, I can't say for sure when it actually melted or relate that to when I started getting the codes) and I'm done trying new ECMs (been through three of them with same end result) until I can get to the sub-harness to replace it and see if that resolves anything. Exceeding the RPM limit/time should be less of an issue with the new McCruise (love it!) that I can just set at 3950RPM (slight red of the 4K line showing just above the needle) and forget about throttling for awhile on the highway. I also need to update my picture...over 151k now.
Click to expand...
Thank you for sharing your experience
 
I ordered a knock sensor sub-harness. My sensor connectors have pretty much disintegrated. Is there any way to replace the harness without removing the throttle bodies?
 
just kind of a response to post 26 - can anyone explain...
the best real world example of back pressure affecting performance I had was with a gradually deteriorating / coroding flexible segment in my 92 Accord I drove it from a slight exhaust leak through to a bad exhaust leak; even then the noise wasn't any worse than some of the aftermarket mufflers so who cares was the rationale
What I noticed after a while was a gradual loss of performance and this was over a considerable period. No fault codes or rough running, just gradually diminished power Got a new flex connector from a wreckers, no tune up no other changes and power was back to normal
 
eyrepb said:
I ordered a knock sensor sub-harness. My sensor connectors have pretty much disintegrated. Is there any way to replace the harness without removing the throttle bodies?
Click to expand...
Brad,

I could be wrong, but I don't think the wires for the knock sensors run underneath the throttle bodies. It's my understanding that those wires run through a large, grey, connector/junction on either side of the bike near where the frame and seat sub-frame come together. I've also heard that it's a real PITA to remove and replace.

If your wires have continuity to the ECM, you might consider just replacing the connectors. Ironically, someone started a thread on these connectors recently. A link to the thread with possible options follows: https://www.st-owners.com/forums/threads/knock-sensor-connector.191396/#post-2595405. Replacing the connectors is likely far less time-consuming and will cost less than replacing the sub-harness. Since I have not replaced the sub-harness myself, I will admit that I don't know the true degree of difficulty. Nevertheless, I wanted you know there was another option.

Chris
 
Thank you Chris. I have had the throttle bodies off/on once and would like to avoid another adventure if possible.
 
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