Reduced engine power.......AGAIN!

[03GMCYukonXL]

2004 Tahoe Z71 fully optioned out.

Ok. long story short, I got the reduced engine power issue, and then I

Replaced the TPS wiring harness: Didn't work
Replaced the actual TPS: Didn't work
Replaced the throttle body: It worked! Been driving like a dream for about a year

Then, just today, the wife called me broken down on the side of the interstate with the reduced engine power and a P2135 and P0120 code. I'm assuming this is a different issue since the truck ran so good for a year. The P0120 code says it's the Throttle/Pedal position sensor A circuit. So can I just replace a sensor or should I try the whole pedal assembly? I have a spare Yukon in my bone yard for parts that has the same adjustable pedals. I'm just kind of down because I literally replaced then engine 6 years ago, and had the transmission rebuilt less than a year ago. I've heard this is a ground issue but I've painstakingly checked EVERY single ground and this thing is solid. No rust. All grounds are solid. I'm ready to sell it but I'm afraid because all the new cars are junk.

 

[S33k3r]

This is most likely a wiring issue. But there is a sensor at the throttle, in the cab. Take a look at it and make sure it is fully snapped in place. But you probably should research those codes further, including Google searching for them, and maybe using some free AI cycles to help. Good luck!

 

[Joseph Garcia]

According to Grok, using your own words:

P0120 (TP Sensor 1 Circuit) + P2135 (TP Sensor 1-2 Correlation) on a 2004 Tahoe with “Reduced Engine Power” points primarily at the throttle body side of the electronic throttle control (ETC) system, not the accelerator pedal.


These codes monitor the dual throttle position sensors (TPS1/TPS2) built into the electronic throttle body. The PCM/TAC sees out-of-range voltage on sensor 1 and/or the two sensors disagreeing. That triggers limp mode. Pedal-related codes are usually in the P212x/P2138 range. Your spare Yukon pedal assembly is unlikely to be the fix unless live data later proves otherwise.


This is a very common failure pattern on 2003–2006 GMT800 trucks/SUVs. GM even issued a bulletin (PIP3089B / Document ID 1869994) covering exactly these codes (P0120, P0220, P1516, P2135) on 2003–2006 Tahoes and related models. The usual root causes are:

• Loose terminal crimps or broken copper strands inside the insulation within 1–4 inches of the throttle-body connector (outer insulation looks fine).
• Poor connection at the throttle-body connector itself.
• Failed internal TPS sensors in the throttle body (sensors are not separately serviceable on these units).
• Less commonly, shared 5 V reference or ground issues (you already checked the obvious grounds).

Recommended troubleshooting order (do this before buying parts)​

1. Scan-tool live data (key on, engine off or running)
   Graph or watch TP Sensor 1 and TP Sensor 2 voltages/angles while slowly sweeping the accelerator from closed to wide-open throttle (WOT) and back.
   • They should move smoothly and stay correlated (the scanner often shows a “TP 1/2 Agree” parameter).
   • Look for dropouts, spikes, flat spots, or one sensor that stops changing.
   • Wiggle the harness at the throttle-body connector and a few inches back while watching the data. If the values jump or the code sets, you have a wiring/connector problem.
2. Physical inspection of the throttle-body harness (most common fix per the GM bulletin)
   • Unplug the 8-pin connector at the throttle body.
   • Inspect terminals for corrosion, bent pins, or looseness.
   • Gently tug each wire at the connector and 1–4 inches back. Feel for broken strands inside the insulation.
   • Perform a loaded voltage-drop test on the 5 V reference, low-reference/ground, and signal circuits if you have a good multimeter.
   • Repair with a quality pigtail (many aftermarket 8-pin throttle-body pigtails exist for these years) or new terminals/crimps. Do not just twist-and-tape.
3. Clean the throttle body
   Carbon/build-up can sometimes contribute. Remove it, clean the bore and plate thoroughly with throttle-body cleaner (do not spray electronics), and reinstall with a new gasket if needed. Perform a throttle relearn afterward if required by your scanner.
4. If data and wiring check out clean
   Replace the complete throttle-body assembly (TPS is integrated). Aftermarket or OEM units are widely available. After install, clear codes and perform the throttle-body relearn procedure (usually key-on engine-off pedal sweeps or a scanner command).
5. Only then consider the pedal
   Monitor APP Sensor 1 and 2 live data the same way. If those look solid and the truck has adjustable pedals, a donor Yukon pedal is fine to try, but it is lower probability with pure P0120/P2135.

Additional notes​

• You already verified grounds thoroughly—good. Still double-check G103/G104 (common on these trucks) with a voltage-drop test under load if the issue persists.
• The TAC module (separate box on the driver’s side firewall area on many of these) is a distant possibility but rarely the first failure.
• Intermittent issues often only appear when the harness is flexed or under heat/vibration, so the wiggle test while watching live data is critical.
• After any repair, clear the codes, do a few drive cycles, and confirm the reduced-power message is gone.

Start with the live-data sweep + harness inspection at the throttle body—that resolves a large percentage of these exact codes without throwing parts. If you can post the TP1/TP2 voltage behavior (idle, mid-throttle, WOT) or photos of the connector, more specific guidance is possible.

 

[03GMCYukonXL]

According to Grok, using your own words:

P0120 (TP Sensor 1 Circuit) + P2135 (TP Sensor 1-2 Correlation) on a 2004 Tahoe with “Reduced Engine Power” points primarily at the throttle body side of the electronic throttle control (ETC) system, not the accelerator pedal.


These codes monitor the dual throttle position sensors (TPS1/TPS2) built into the electronic throttle body. The PCM/TAC sees out-of-range voltage on sensor 1 and/or the two sensors disagreeing. That triggers limp mode. Pedal-related codes are usually in the P212x/P2138 range. Your spare Yukon pedal assembly is unlikely to be the fix unless live data later proves otherwise.


This is a very common failure pattern on 2003–2006 GMT800 trucks/SUVs. GM even issued a bulletin (PIP3089B / Document ID 1869994) covering exactly these codes (P0120, P0220, P1516, P2135) on 2003–2006 Tahoes and related models. The usual root causes are:

• Loose terminal crimps or broken copper strands inside the insulation within 1–4 inches of the throttle-body connector (outer insulation looks fine).
• Poor connection at the throttle-body connector itself.
• Failed internal TPS sensors in the throttle body (sensors are not separately serviceable on these units).
• Less commonly, shared 5 V reference or ground issues (you already checked the obvious grounds).

Recommended troubleshooting order (do this before buying parts)​

1. Scan-tool live data (key on, engine off or running)
   Graph or watch TP Sensor 1 and TP Sensor 2 voltages/angles while slowly sweeping the accelerator from closed to wide-open throttle (WOT) and back.
   • They should move smoothly and stay correlated (the scanner often shows a “TP 1/2 Agree” parameter).
   • Look for dropouts, spikes, flat spots, or one sensor that stops changing.
   • Wiggle the harness at the throttle-body connector and a few inches back while watching the data. If the values jump or the code sets, you have a wiring/connector problem.
2. Physical inspection of the throttle-body harness (most common fix per the GM bulletin)
   • Unplug the 8-pin connector at the throttle body.
   • Inspect terminals for corrosion, bent pins, or looseness.
   • Gently tug each wire at the connector and 1–4 inches back. Feel for broken strands inside the insulation.
   • Perform a loaded voltage-drop test on the 5 V reference, low-reference/ground, and signal circuits if you have a good multimeter.
   • Repair with a quality pigtail (many aftermarket 8-pin throttle-body pigtails exist for these years) or new terminals/crimps. Do not just twist-and-tape.
3. Clean the throttle body
   Carbon/build-up can sometimes contribute. Remove it, clean the bore and plate thoroughly with throttle-body cleaner (do not spray electronics), and reinstall with a new gasket if needed. Perform a throttle relearn afterward if required by your scanner.
4. If data and wiring check out clean
   Replace the complete throttle-body assembly (TPS is integrated). Aftermarket or OEM units are widely available. After install, clear codes and perform the throttle-body relearn procedure (usually key-on engine-off pedal sweeps or a scanner command).
5. Only then consider the pedal
   Monitor APP Sensor 1 and 2 live data the same way. If those look solid and the truck has adjustable pedals, a donor Yukon pedal is fine to try, but it is lower probability with pure P0120/P2135.

Additional notes​

• You already verified grounds thoroughly—good. Still double-check G103/G104 (common on these trucks) with a voltage-drop test under load if the issue persists.
• The TAC module (separate box on the driver’s side firewall area on many of these) is a distant possibility but rarely the first failure.
• Intermittent issues often only appear when the harness is flexed or under heat/vibration, so the wiggle test while watching live data is critical.
• After any repair, clear the codes, do a few drive cycles, and confirm the reduced-power message is gone.

Start with the live-data sweep + harness inspection at the throttle body—that resolves a large percentage of these exact codes without throwing parts. If you can post the TP1/TP2 voltage behavior (idle, mid-throttle, WOT) or photos of the connector, more specific guidance is possible.

Thank you,

The problem is, I've already replaced the pigtail harness, AND the entire throttle body with a GM OEM unit. After I did that, the truck worked perfectly for almost a year. Not a single issue. It just randomly popped back up for no seeming reason.

 

[Joseph Garcia]

Thank you,

The problem is, I've already replaced the pigtail harness, AND the entire throttle body with a GM OEM unit. After I did that, the truck worked perfectly for almost a year. Not a single issue. It just randomly popped back up for no seeming reason.

You could walk through the troubleshooting procedure with a quality bi-directional scanner, as suggested, if you have not already done so.

 

[Fless]

No fix is permanent, so it could be caused by a similar issue. Follow the data.

 

[LsHart]

Synced?

 

[03GMCYukonXL]

I did the throttle body relearn procedure if that's what you mean.

 

[03GMCYukonXL]

You could walk through the troubleshooting procedure with a quality bi-directional scanner, as suggested, if you have not already done so.

I don't have one of those. I do have an HP Tuner though.