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Honda 225 Oxygen Sensor Heater

KingsPointer

New member
Hello,

I have a 2005 BF225 on a 24 Proline that has been giving me some issues.

Initially I had a bad VTEC sensor (fault code 21). After replacing that I put 10 hours on the motor before I got another alarm, this time fault code 21. I swapped out the oxygen sensor with a different (used) one and fired the engine back up, but immediately got an alarm (this time fault code 41). Thinking both sensors were bad I bought a new O2 sensor, to no avail.

I've checked the voltage of the heater wires at the sensor connector and get a steady 14.4 volts. I've disconnected and cleaned all connections at the ECM (though they weren't corroded) and nothing; the alarm comes on 6 seconds after starting the motor. Any ideas what the problem could be? Would a bad ECM or short in the wire still give a solid reading at the connector?

Any help or ideas are greatly appreciated.
 
Did you reset the ECM to clear codes and then still get a code 41?

Other causes for a code 1 or code 41 are misfiring spark plug, failing coil, or low fuel pressure. But you say that otherwise the engine is running fine, so that seems remote.

If I were in your shoes, I would repeat the process of resetting the ECM to clear codes and make sure they are cleared before restarting the engine. Then run the engine until I get the alarm, then re-check for codes.
 
I clear the codes every time I swap out the sensor, disconnect any connections, etc. Maybe I am barking up the wrong tree with the O2 sensor but it seems strange that there are no issues with the way it runs if there is a fuel/spark problem.
 
Typically, when the HO2 sensor fails, your fuel mixture is too rich and fuel economy goes down. The engine may seem to run fine. You might want to check your plugs for any black soot. Typically, they should be milk chocolate brown if igniting properly.
 
Yes that's why I initially thought it was the O2 sensor that was bad, the service manual says that it will revert to a pre-programmed mixture. So was the new sensor I got faulty? Or is it a problem with the ECM? I am tempted to get the DR H diagnostic from BRP but I'm not sure it would help in this case. I guess I might as well change out the plugs and check compression, its probably well overdue.
 
The problem with checking voltage or resistance on wiring and connections is that a poor connection or a partially broken wire might deliver expected voltage and show reasonable resistance but not be able to carry the required amperage to operate the circuit. This might be particularly true of the O2 heater circuit which requires a relatively high amp load.

When in doubt about circuits like this it is best to disconnect them at both ends and load them to check their operation. Most simply apply 12v to one end and use the other end to power an automotive type headlamp like an old "sealed beam" and then ground it. This applies around 3 to 5 amps load which is sufficient for testing most 12v circuits. A brightly burning lamp indicates good amps flow and the circuit can be tapped, shaken and pulled on to detect any intermittent loss of current flow.

In this particular case, you can power a lamp with the voltage you have present but you need to insure that the ECM is safely out of the loop. If the heater grounds through the ECM, you should provide a remote ground to the battery.

Just sayin'.

Good luck.
 
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To be honest, I have never measured the amps on an O2 hego circuit. I'm guessing somewhere in the neighborhood of between 1 and 3 amps but that's just a guess. But even if it's higher, the headlamp test should be a good indicator of a reliable circuit if it burns bright and steady as the wiring is manipulated and the connectors are pushed and pulled on.

Good luck.
 
So today I was able to get up to the boat and try load testing the circuit. First I tested just the hot wire on its own, and found no problems. Playing with all the connectors and wires made no difference in the brightness of my test light. Next I tried both the hot and the ground. I traced the ground back to the first connection I could find and disconnected it; this was a flat connector located on top of the starboard head. Again the load test showed the circuit to be fine. It seems to me that the circuit is sound, so is the ECM bad?

My testing led to another problem... in trying to disconnect the ground I dropped the plastic clip (from on top of the starboard head) overboard. Is there anywhere that I can get a replacement? I can't even find it in any of the parts diagrams.
 
Ok so the connector I am looking for is listed in the service manual as "junction connector [24p]". Any idea of a part number or where to get one?

Also, is there any source for the service bulletins? Particularly #56,70, and 71. Today I got to wondering if the new sensor (serial number ends 013) wasn't compatible with my ECU (serial ends 023), but best i can tell it is. I also ordered a copy of Dr H because I feel like I have to be missing something obvious, so hopefully that will help me figure it out.
 
Maybe try and connect the old O2 senser and cycle power and see how quickly it heats up. Getting a bad new part is entirely possible. Just run the motor with the old senser grounded but not in the engine and see if you get a different code.
 
Something that sometimes causes "phantom" type codes is a faulty rectifier set in the alternator. If there are bad diodes it can cause a scenario that can have an ac electrical signal riding on your 12v dc supply to the ECM and sensors. It's called "ac ripple voltage". Naturally, the computer doesn't like it at all and it can cause problems that don't seem to make any sense.

The good news is that it is very easy to detect with a good digital voltmeter. You simply put the meter on the ac scale and probe the battery + and - terminals while the alternator is charging. Ideally, there should not be any ac signal to detect. A reading of 0.025 ac volts or less could be normal but my experience is that I like to see 0.002 vac or less on a "clean" charging system.

Just something quick and easy to check I wanted you to have. Maybe your Dr H will help when it comes.

Good luck.
 
Yes, I like that one too....although 99 out 100 times that's not the problem......it IS a problem once in a great while and it's just too quick and easy to check not to eliminate it as the cause.

Ignition coils failing cause a higher percentage of the unexplained codes these days with the coil over plug (COP) failures being as frequent as they are. But, without an oscilloscope and, sometimes even if you have one, they are much more difficult to identify.
 
Thanks for the suggestions. I am going to pick up a better multimeter today as the cheap one I have isn't accurate enough.

I was finally able to hook the motor up with Dr H today. Voltage and current to the sensor show as good, with the draw being around 1.85 amps. I ran through the flow diagram that came with the Dr H which suggested that either my sensors are bad (I doubt this but I couldn't get a solid resistance reading - thus the need for the new multimeter) or problems with the ECU. Hopefully tonight i will be able to test all the sensors again and I am going to try to get my hands on a used ECU to try swapping them out. I will also try the suggestion about the phantom AC voltage.

I noticed in Dr H there is an item "HO2S Control" that was shown as off. I couldn't find anything about it in the manual, can anyone explain what exactly this is?

Thanks for all the suggestions, I really appreciate the help.
 
I'm speculating here, but the HO2S needs to have current on via the ECU. When the key switch is off, the heater element will also be off, thus the "off" reading. Use your multimeter to check that the HO2S heater circuit is getting 12 V with the key switch on.
 
I'm speculating here as well, but if it's not a power to sensor thing as chawk suggests, I'm wondering if it might be a descriptor for what is known as "closed loop operation"?

Computer controlled fuel systems operate in two distinct modes....open loop....usually when the engine is first started, is cold and fuel enrichment is required to keep it running. The computer will ignore the O2 sensor feedback during this time and fuel the engine according to a program to allow a rich condition to exist.

When a predetermined amount of time has passed after engine start and/or the engine has reached a target temperature, then the computer will enter closed loop operation and will meter the fuel according to O2 input. This COULD be described as O2 control.

Most scanners I've used simply say open loop or closed loop but this may be a description unique to Honda???

Just thought you could look for a possible correlation between "control-no" and "open loop" or "closed loop"
 
Hopefully the photo worked; the item I am talking about is just visible in the bottom of the window. The open/closed loop explanation makes sense. I guess tomorrow I need to pay better attention when I first start the motor and see if it changes, I think the motor would have been sufficiently warmed up by the time I took this snapshot.
 
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Yes, the screen shot came through. In my automotive diagnostic world each of those lines of information is known as a PID for Parameter IDentifier. See if you can find a PID description of open or closed loop. If you don't then that line for control is probably Honda speak for loop.

I'm assuming that you highlighted the O2 voltage PID on purpose since 3.78v is a long way away from the 12v or 14.4 volts the heater should be seeing. This would indicate a voltage drop is taking place in that circuit. Google voltage drop testing, read a couple of the tutorials and when you have your new meter, you can go on a hunt for exactly where it is. That looks to be the problem from here. Without a wiring diagram all I can say is that the first place to look might be the relay contacts that power the heater. They are usual suspects.

Good luck.
 
Hmmm. Just went back and reread your first post. You stated that you measured 14.4v at the sensor. So that means the relay is delivering what it should but the ECM is only seeing 3.78v.

Here's where I would like to see a wiring schematic to see how the ECM takes that measurement.
 
I beleive that the voltage is that created by the O2 sensor, however it is interesting that it is so high (it does drop to ~1V when the engine is hot). Also notice there is no IAC current or alternator voltage... I didn't even notice this yesterday. I went back this morning and took a closer look at everything with the motor running. My understanding is that the info I posted is that saved by the ECM as soon as the DTC is stored. So it seems the oxygen heater code is tripped as soon as the ignition is turned on, before the motor is even started. Now that should rule out the suggestion of stray AC voltage, although I did check and measured less than .02 volts.

Here is the instantaneous info with the engine running. I let it heat up and noticed no difference with the HO2S Control PID, it always remained "OFF". Out of curiousity I disconnected the sensor with the motor running; the voltage and current dropped as expected but otherwise there was no change. I also found it curious that I didn't get any other codes for disconnected oxygen sensor when unplugging, only code 41-2 for the heater. I picked up a spare wiring harness and a new (used) computer is on the way so I will try changing those out next week.


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I used to own a factory HDS but had to sell it when the dealer I worked for lost his franchise. I couldn't get updates and I didn't work on anything larger than 20hp so I didn't use the HDS. But, being technical geeky I did read the manual and I took a lot of notes which I have hung on to.

The alternator or ACG (for ac generator) PID has a value of 0 to 5v. This is taken from the F terminal and is used by the ECM to monitor electrical loading on the charging system so that the IAC can be adjusted to maintain consistent idle rpm.

Charging voltage is monitored at the battery. So, 0 volts on the alternator PID simply reflects very low to no current flowing in the electrical system. I'm not sure why the IAC was showing 0 as well. There's probably a logical explanation but I don't have it.

You may want to see if your copy (BRP?) has a tools button that you can go in and change values from voltage to degrees Fahrenheit or Celsius for temp sensors, IAC to percentage of command and TPS to degrees of angle. Map and Baro may also display in in. Hg . This is easier to read directly instead of having to consult voltage conversion charts.
 
Correction:
the ACG voltage increases when the electrical load is lowered and decreases as the load is raised.
Had it donkey/horse backwards.
Apologies.

Other things that look odd on that first screen shot are:
Injector pulse width looks pretty high for idle. I have reference at 2.6 to 3.8ms at idle

Then, when you look at your last screen, the inj pw is closer to normal idle but oil pressure low says on and acg lamp says on???
 
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Thanks for the notes, I appreciate the insight. The units that you mention can easily be changed via the drop down menu, I just neglected to do it before I took the snapshots. I guess that's downside of figuring out how to use Dr H on my own.

As far as the oil pressure light and ACG lamp... good questions. Hopefully the low oil pressure indicator is because the motor was just started, as I never got any alarm. I will be sure to check the light next time and see if it stays on.

For the injector pulse width, just speculating but I am guessing that the difference is due to the fact that the value in the first picture is related to the code, not an instantaneous readout.

And lastly for the ACG lamp... I have no idea. Seems like the voltage is acceptable so I am not sure why the lamp would be on. Again I guess i will check and see that the light is actually illuminated because I haven't noticed that it was.

Thanks again for the advice and suggestions, you guys have been a lot more helpful than my local dealer.
 
Looking through my notes.....the ACG lamp PID will say ON when the alternator is properly charging...so yours is normal.

The EOP LOW will say ON if the oil pressure is within the normal range at low rpms....yours is normal

The EOP HIGH will say ON if the oil pressure is within normal range when the rpms are above 3000. At lower speeds it will be OFF. Since the screen shot was taken at idle, yours is normal.

The H2O2 Control PID:
This PID is as chawk _man first suggested. It simply indicates whether or not the O2 heater is being powered. The heater receives power until the sensor reaches the prescribed temperature. Once it is hot enough, the ECM shuts off current to the heater and the PID will change from ON to OFF. So, again, your screen shot looks normal for a warmed up engine.

HO2S CURrent is listed as 0-2.000A

The voltage produced by the sensor listed as HO2S is 0-1V. Ideally it constantly ranges from 0.2V to 0.9V This makes the 3.78V reading for that PID suspect and could indicate an internally shorted sensor or a short to power in the sensor wiring.

Sorry for doing this so piecemeal but my memory isn't as good as it used to be.
 
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No problem, thanks for the info. I hooked up the ECM i bought today and ran the boat a little bit. No alarms at all. This new ECM is well used; it has over 4100 hours on it, a testament to the life of these motors I suppose. So it seems like somehow the old one was fried. I only ran Dr H when I first fired the boat up and immediately noticed one difference; instead of the current holding steady at 1.85 amps it continually bounced between 0 and .75, like it was cycled on and off. Not sure if that is how it is supposed to work or not, but after 30 minutes of running I still didn't have any alarms.

I plan on using the boat some more tomorrow and will hook up Dr H at the end of the day for a more thorough check. Thanks again for all the help and suggestions.
 
Just as an aside, I've been on this forum since at least 2009 and your discovery is one of the very few bad ECM's I can recall. Normally, they are very reliable.

Keep us updated.
 
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