I have a 2002 Trailblazer LTZ I-6 4WD 143k miles. When starting after it has been sitting for a while (like overnight) the Service Engine Soon and Reduced Engine Power lights come on. While idling with the REP light on the RPM will vary back and forth from 600 rpm (idles very roughly at this point, almost to the point of stalling, and spits water from the exhaust pipe) to 1200 rpm (idles fine). If I let the truck idle for a while (anywhere from 10-20 minutes), shut it off and restart it, the REP light clears and the truck idles and drives normally, however the SES light is still on. However, after the truck has been driven some distance, when turned off and restarted, the SES light will go out. It seems that the colder the outside temp the longer it has to run to clear the REP light. When I am able to drive the truck, with the SES light on and the REP light off, the OBD2 reads P1515 and P1516 (thanks to my local Auto Zone) which are both throttle position sensor codes. The work done so far is cleaning the throttle body, replacing the throttle body/TPS sensor (which is all one unit), and replacing the accelerator position sensor. Still having the same problem. I should add that I had this problem last winter and didn't get around to fixing it. When the weather got warmer the problem went away and I was hoping that was the last of it. But now that the nights are getting colder the problem is back. Any ideas?

 

Welcome to the forum. I don't think P1515 and P1516, in the absence of other codes, would have anything to do with the APP sensor. You may want to check for something that prevents the throttle plate from closing or moving freely. Disconnect the electrical connector from the TB and then try moving the throttle plate by hand. Also, there may be bad crimps inside the electrical connector responsible for resistance changes in the circuits from temperature and moisture variations:

Terminal G, GRY wire, TP SENS 1 5V REF from PCM connector C3, terminal 3
Terminal C, BLK wire, TP SENS 1 LOW REF from PCM connector C3, terminal 59
Terminal A, DK GRN wire, TP SENS 1 SIG to PCM connector C3, terminal 28
Terminal B, LT BLU/BLK wire, TP SENS 2 5V REF from PCM connector C3, terminal 49
Terminal H, BLK/WHT wire, TP SENS 2 LOW REF from PCM connector C3, terminal 16
Terminal D, PPL wire, TP SENS 2 SIG to PCM connector C3, terminal 30
Terminal E, YEL wire, TAC MTR CTRL 1 from PCM connector C3, terminal 26
Terminal F, BRN wire, TAC MTR CTRL 2 from PCM connector C3, terminal 25

 

Thanks, will take a look at the sensor voltages. The throttle body and sensor, as well as the accelerator position sensor, have been replaced with new GM parts. It was suggested elsewhere that there could be a problem with the 5V reference which is shared by the fan clutch. I was advised that the clutch (which is original, surprise!) or the harness to the fan clutch could be causing reference voltage fluctuations. The suggestion was made that I disconnect the connector from the fan clutch and see if the problem goes away. Disconnecting the fan clutch connector did not change anything. But since the problem seems to be temperature-sensitive I was leaning towards the sensors or (heaven forbid) the PCM itself.

 

I am thinking that I should park the truck overnite at my local Auto Zone and have them read the codes when the truck is first started and in REP mode. After the engine has warmed up the REP light goes off but the SES light stays on, and I have been finding the P1515 and P1516 codes. I wonder if there may be other codes displayed when the truck is first started and in REP mode that clear when the REP clears? Comments?

 

Or, invest in a scan tool. Nowadays you can even get one with Bluetooth that can interface with smart phone apps.

 

Well I got the truck scanned after it had been parked overnite and not run. Wound up replacing the PCM itself. Finally fixed the problem. Thanks to all who commented on this thread!

 

Congrats and thanks for the update.