Hi all,
I have the above unusual error stored in the ECM of my 1990 Chevrolet Caprice.
When I clear the memory, the error disappears, but as soon as I start the engine, code 31 is immediately set.
When the engine is cold, the check engine light stays on for about 5 minutes and then goes off.
When starting warm, the light sometimes goes off after just 30 seconds.
The strange thing is that, according to the repair manual, code 31 shouldn't even exist.
However, since this is one of the last models from the 1990 production year, could it still be possible?

Does anyone have an idea what I should check?"

Greetings from Germany,
Micha

 

Unfortunately my library of CSMs doesn't reach to 1990.

I have found some evidence that for 1990, Code 31 on the 307 cars was a fault in the Park/Neutral switch. Take that with a healthy dose of skepticism as I cannot verify the information from factory sources. That switch is located on the top of the steering column near the floor and looks like the old neutral safety switch. FYI, these cars don't have a neutral safety switch, as there is a mechanical blocking ring in the steering column that physically blocks the lock cylinder from turning to the START position unless the shifter is in PARK or NEUTRAL.

I have also found at least one citation that Code 31 is a fault in the knock sensor. The 1989 and 1990 307s were the only Olds motors to use a knock sensor. Again, this info is not verified.

 

I just noticed this post. I will look at my old Snap On scanner in the morning. It has a book with the codes.

 

Micha,
CODE 31, CANISTER PURGE SOLENOID VOLT SIGNAL
INCORRECT

I you can send me your e-mail address I can send you the troubleshooting procedure.

 

Thanks Joe and Bill for your Ideas - i will check on weekend.
The previous owner removed the entire air system—I recently tried to figure out whether he did everything correctly—there's a nice post about it here in the forum.
I think it said that the CANISTER PURGE SOLENOID was not needed.

Fortunately, I have a bag with all the small vacuum cans that were removed, so I can test it out.
I'm also surprised that I suddenly had the error in the memory without any changes.

I´ll let you know my results soon.

Greetings, Micha

 

Hey guys,
thanks again for your tips. I've been living with this fault for so long that I had almost given up hope.
I found the valve in the basement and simply made the electrical connection—without any vacuum hoses.

The fault is actually gone now—I'm thrilled...

Greetings, Micha

1990 Chevrolet Caprice Station
1995 Chevrolet Astro (Starcraft)
2005 Audi A3 (For insurance purposes only—in Germany, you are not allowed to drive only classic cars older than 25 Years.)

 

You need to not only check with a multimeter but also ensure that the correct components are installed in the car. Sometimes, after purchasing a car, it turns out that a different transmission or module is installed, and this is where strange glitches begin.
First, check for discrepancies using the find transmission by vin number at https://carfast.express/en/window_sticker . This will save time when diagnosing the electronics.