DTC P1639 & P0122

trentw · November 20, 2005

Hi,

I've got a problem with my 2001 Chev Tahoe Z71, 5.3L.

The CEL illuminated and is on steady. I've retrieved two codes: P1639 & P0122.

The only symptom other than the CEL light is that the vehicle fails to accelerate as forcefully as it used to. Even when at slow speeds the accelerator is fully depressed, the transmission fails to shift down and the engine does not seen to exceed 2000rpm unless I manually shift down. The sluggishness is uniform throughout the range of the accerator.

I'm leaning towards replacing the TPS given the symptoms. I've been unable to duplicate code P0122, but when cleared, P1639 consistently shows up after two test runs(as it should).

Any suggestions? Thanks in advance for your time.

Cheers,

Trent

GM-Tech · November 20, 2005

P1639 is a problem in the 5 volt reference circuit for the TPS and the fuel tank pressure sensor. If the fuel tank pressure sensor pulls down the 5 volt circuit, it's going to cause the TPS fault.

I've seen some pressure sensors with this problem. I've never seen a TPS cause this problem. It could also be a wiring issue.

trentw · November 20, 2005

I guess I'll start with the pressure sensor and wiring then. Thanks for the quick reply.

GM-Tech · November 20, 2005

I guess I'll start with the pressure sensor and wiring then. Thanks for the quick reply.

I hate guessing. It can cost you money to put on a part that doesn't fix anything. Do you have a DVOM? I'd pull the connector to the TPS and measure the 5 volt reference there and see if you have 5 volts (or nearly so). Of course if whatever is causing the problem is not causing it at that moment, this may not tell you much, and of course if it is the TPS, this wouldn't tell you anything either, since it would be disconnected. You could backprobe while it's connected, but be careful not to damage insulation, weather seals, etc.

Since you knew those codes, does that mean you have access to a scanner?

trentw · November 21, 2005

After checking all connections related to this circuit my CEL cleared and I've been unable to cause the problem again. The truck drives great with no symptoms. It would seem there was a bad connection somewhere. Thanks for your help guys.

Cheers,

Trent

trentw · November 22, 2005

Well, the code and light are back. I'll document what I've done in case anyone else runs into this.

1) I started by checking the TPS. The reference voltage was only 3.3V, not the required 5V.

2) I then pulled the PCM and checked the resistance of the wires running to the TPS, they were fine.

3) I then removed the the fuse box and located the connecter that runs to the ABS and Fuel Pump/Sensor(blue connector on a '01 Tahoe). I unplugged it. This obviously caused errors with the ABS and fuel pump but cleared the P1639 code and the reference voltage returned to 5V at the TPS. I plugged the ABS and fuel pump power back in.

4) I put the truck on a lift and had to drop the fuel tank to unplug the fuel sender.

5) When I powered up, everything else worked fine and my TPS reference voltage remained at 5V.

6) I kicked myself for not starting here as the GM-Tech wisely suggested. I then put everything back together.

As was earlier suggested, it seems that the fuel pressure sensor is pulling down the voltage on the TPS, causing the CEL light to illuminate.

My next question is:

My local dealer says I need to replace the fuel pump($600 ) because everything in the fuel tank comes as one piece. I ordered the pump but I'm left wondering if my fuel pump needs to be replaced just because of a bad sensor?

Thanks again,

Trent

GM-Tech · November 22, 2005

My local dealer says I need to replace the fuel pump($600 ) because everything in the fuel tank comes as one piece. I ordered the pump but I'm left wondering if my fuel pump needs to be replaced just because of a bad sensor?

It should be available separately, but I could be wrong for your particular vehicle. Give me a full VIN and I'll have our parts department look it up.