I have a 99 V6 with the drive by wire throttle body. Its at 104K miles in the last 3 months it received: plugs, wires, timing chain, cam sensor, crank sensor, oil pressure sensor, fuel pressure regulator, and fuel filter. It was running well and all of a sudden I have a barely start or no start condition. When it starts its missing badly and the response to throttle input by popping and killing. When I attach a scanner stored codes are :
P1120-99 Throttle position sensor circuit 1
P1220-99 Throttle position sensor circuit 2
P1221-99 Throttle position sensor 1-2 correlation
P1516 -99 Command vs actual throttle position performance

I have cleared codes and they return if I'm able to get it to start for a second.

If I turn key on and depress the accelerator pedal the throttle body valve follows pedal position smoothly to wide open. I have verified the connector ref voltage for both TP sensors is approx 4.3 Vdc. I have varied as the accelerator pedal is pressed the TP1 and TP 2 sensor output is swings from approx .5v to approx 3.1 Vdc. Have verified both TCS fuse ETC fuse are both good. It seems to me that the throttle body is working in spite of the codes. Using a Xtool A30M to read codes and live data. I have tried to follow the OEM manual P1120 troubleshooting but it is really tough with a third party non GM Tech scanner.

I'm starting to wonder if it could be the cam or crank sensor but I don't understand why it wouldn't set different codes in that case.

Any ideas?

 

can you read the live data while engine is running or turning over trying to start? see if its jumping around. sure sounds like lose wire or connection somewhere.

 

It runs for only a second or two and then dies. Even with recorded data I don't get enough data to really see anything.

 

Finally resolved this issue. After all of the Drive by wire TB sensor data looked solid I decided to get to basics and ignore all of the TB codes. I verified I had spark, Although I could hear the fuel pump run when the key was turned on but when I attached a pressure gauge at the FI rail I only had a weak 5psi. Car already had a new fuel regulator so I moved the gauge back to the inlet side of the fuel filter. Still hear the pump run but only 5 psi. No fuel no start. I removed fuel sending unit and the plastic line between the pump and the outlet fitting cracked in two places when I tried to remove it. I suspect the pump was working and the line was cracked. I put a new pump / line assembly. Car starts and runs well again no codes. Computer was dropping codes that were indirectly related to the real issue. Before you buy a use TB verify pump pressure!!!

Basics still apply you need compression, spark, and fuel to start an engine. Always verify the basics.

 

yep there is no code for low fuel pressure,,,, lol. good deal. have fun.

 

Thanks to everyone who replied to my initial post.