2004 Cavalier - Engine Check - Won't use CNG

[jpchuk]

I bought my 2004 bi-fuel cavalier in AZ. I flew out from Medford Ma to pick it up and drive it back. I had the seller add a second (3 gal. equivalent) cng tank. Since owning this issue i will relate has repeated itself 4 times with 4 different machanics.
The check engine light comes on and when it does, the system no longer recognizes that cng is in the tanks - it runs on gasoline only. Each mechanic has found the DTC and it is P1431.
When they erase it, the system goes back to operating correctly and that will last for a varying amount of time - maybe 2 months maybe 2 weeks.
The last mechanic, who says he is a CNG tech, at Liberty Chevrolet in Wakefield said there were no leaks he could detect, he says he traces it back to the fact that there is one sensor on the original tank fill tube but two tanks. But that doesn't make much sense because I can fill both tanks (i fill about 8 GGE, factory original tank is about 5 gge) and use both up and this repeats many times. Then I get this check engine light and P1431.
Any ideas?

[siai47]

Re: 2004 Cavalier - Engine Check - Won't use CNG

P1431 sets when the vehicle is operating on CNG and after after running for a period of time there is no change in fuel pressure from the fuel pressure transducer. Your dealer might be on the right track. How is the aux tank plumbed into the system? Can you select between tanks bypassing the factory tank? If so, this is the problem. The factory tank must be in the system and the pressure and temperature transducers must be recording the use of fuel from both tanks at the same time. If the aux tank is draining into the factory tank then you shouldn't get this DTC. The other issue is the possibility that there is a short or open in the wiring going to the pressure transducer. You need to check the wires and connectors. You might have a bad pressure transducer (stuck) at a certian pressure. Finally, with the extra fuel, the pressure change might not be occurring fast enough to satisify the ECU programming

[freedml]

Re: 2004 Cavalier - Engine Check - Won't use CNG

Maybe one other possibility -- increasing the effective tank capacity may cause the pressure to change too slowly to be properly recognized by the ECU (especially if in conjunction with a temperature change in the wrong direction). I would isolate the new tank and see if it keeps happening when using just the stock tank.

[dmb5]

Re: 2004 Cavalier - Engine Check - Won't use CNG

JPCHUK,
Did you resolve your P1431 error with your CNG Fuel Pressure sensor?

[Pclement]

Re: 2004 Cavalier - Engine Check - Won't use CNG

My cavalier does this about 1-2 times a year. The Gus paulos wanted 100 bucks to check the system even though they couldn't tell me what the code even meant. Anyway, nothing was and has ever been wrong with the cng system on my car. My solution was to buy the cheapest OBDII reader and just reset the code every time. It's a little bit of a pain but better than asking my mechanic to reset it every time.