Notes on Diag 31 for Gen1

On the way to Little Big Horn battlefield with Fontanaman last week, I was suddenly confronted with a check engine light while cruising down I-90. It turned out to be code 31, and there was very little available on internet regarding this code, so I'm adding my experience to the collective.

Error code 31 happens when the closed loop fuel injection system runs out of bounds (always reading very lean). The manual lists the following causes:

1. Open or short in the wiring harness

2. Fuel pressure too low

3. Clogged injectors

4. Defective O2 sensor

5. Malfunction in the ECU

6. Other malfunction

Based on some other posts on this forum, it is reasonable to remove item #3 from this list (thanks Dcarver), since powdered JB weld won't throw the code.

If you've been messing with your bike, and have had the tank up or off, or the right side fairing, etc., then you might want to look into #1 to find any pinched or broken wires.

#2 and #5 should be accompanied by some serious running issues.

In my case, I didn't notice anything other than the light itself. Back at the hotel, Big John was kind enough to point out that an O2 sensor is a heat activated source of voltage, and sure enough, I couldn't even get 0.1V out of mine in the parking lot. Note that if I had been able to generate voltage from the sensor with it unplugged from the harness, but then saw no voltage when back probing it while plugged in, then you would need to check out #1 in finer detail. Seeing appropriate voltage under both conditions would make me want to back probe the ECU where the O2 sensor connections are.

Today, upon replacing the sensor and locking the throttle at 2500 RPM for five minutes, the light went out and the code went away.

That's interesting since I don't even have an 02 sensor (removed when I installed the PC3) and have never had the check engine light or code 31 appear.

Yeah, I got a lot of offers for trading O2 sensors from bikes with power commanders, but a gen1 sensor was elusive (they are a 2-wire unheated sensor).

When fuel mixture is rich an O2 sensor will generate up to 0.9 volts, when the mixture is lean it will drop to about 0.1 volts. A fully warmed up O2 sensor will cycle up and down with a mean of 0.45 volts. O2 sensors have to be designed to meet OBDII specifications, one characteristic is a very specific voltage profile.

It occasionally happens that a code 31 will pop up for no apparent reason and if the code pops up it is commonly after a long steady run in hot weather. It is almost never a hard failure, it seems to be a transient event that turns on the light. If you had a condition that caused a hard failure you would have driveability issues. Via diAG, reset the code, clear history and you probably won't see the light or the code again.

The code was not clearable in my case, as I tried to wipe it on the side of the road and drive on. It also couldn't be cleared once the new sensor was in, I had to wait for the bike to warm up, then run it at steady cruising RPM's for several minutes to extinguish the light.

I lost about 3 mpg from having no sensor, but the bike still ran just fine. I suspect the ECU is plenty capable without the feedback loop being in place.

audiowize said:

...I suspect the ECU is plenty capable without the feedback loop being in place.

Click to expand...

Ask anyone with a Power Commander, the Gen I & II PC says to unplug the O2 sensor. The O2 sensor is not part of the basic three inputs that the ECU uses to determine the 3D Speed Density injection profile, it is one of 6 small trim elements the ECU used to optimize emissions. The injection duration is based on the basic injection duration obtained through the throttle position, intake air pressure, and engine speed to which injection duration compensation based on the signals from various sensors such as the intake temperature sensor, atmospheric pressure sensor, and O2 sensor is added to determine the final injection duration.