What causes a Gen III LS-swap drive-by-wire system to enter limp mode at wide-open throttle with P1125, P2120, and P2125 despite apparently correct pedal and TPS voltages?
The pedal, TAC module, and throttle body are mismatched years. Gen III drive-by-wire parts only work as a matched set — 2003-2005 systems share parts, but 2005 switched the pedal to a 2-pot sensor and 2006+ pedals are completely different and not interchangeable — and the tune in the PCM has to match the throttle hardware too. Voltages can look right at idle and part throttle while the APP/TPS correlation tables disagree at WOT, which is exactly when the PCM throws P1125/P2120/P2125 and drops into reduced power. Get a pedal, TAC module, and throttle body all from the same year range (or one donor vehicle) and flash a tune written for that throttle system.
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Who this applies to
- Computers: 0411 (P01), P59
- Engines: LM7, LQ4, LQ9, LS1 (Gen III truck/SUV DBW)
- Transmissions: 4L60E, 4L80E, T56 (trans-agnostic)
- Not for: Gen IV E38/E67 (no TAC module — pedal wires to the ECM); cable-throttle swaps
HP Tuners/EFILive or stock tune — hardware-first fix (works regardless of tuning software)
- Replace the DBW parts as a matched set: pedal, TAC module, and throttle body from the same year range (2003-2005 together; 2006+ together — never mixed). matched year-range pedal + TAC + throttle
- Flash or verify a PCM tune written for that throttle system — the tune must match the hardware. PCM tune matched to throttle system
- Ohm and wiggle-test the APP/TAC harness end to end; repair any broken wire inside the loom. harness continuity confirmed
- Clean and reseat the TAC connectors (dielectric grease on the pins); confirm TAC ground integrity, then clear codes and do a WOT pull. connectors serviced, grounds verified
Careful
- Do not drive it hard in reduced-power mode to 'test' — the PCM is limiting throttle for a reason, and a stuck throttle disagreement at WOT is how you get an unintended-acceleration scare.
- If you replace parts one at a time hoping for a fix, you can burn through three TAC modules and still have the codes — match the set first, then chase wiring.
More in the swap tune checklist, or build a checklist for your swap with the tune checklist tool.
Go deeper
Check this first
- Identify the year of every DBW part: pedal assembly, TAC module, throttle body, and PCM OS. A 2006+ pedal on a 2003-2005 system is the classic mismatch.
- Check the TAC/APP harness for a broken wire inside the loom — one documented case of un-clearable P2120/P2125 was exactly that.
- Log actual vs desired pedal position and TPS %; if the APP/TPS readings don't match, the PCM sets codes and goes into reduced power. Also verify TAC grounds.
Try this, then this
- Match the parts: swap in a pedal, TAC module, and throttle body all from the same year range as the PCM tune. — Year mismatch is the most common cause and the cheapest real fix — voltages can read fine while the correlation tables disagree. Worked if: Codes clear and stay cleared through WOT pulls.
- Wiggle-test and ohm the APP/TAC harness; repair broken wires inside the loom. — A documented un-clearable P2120/P2125 case was a broken wire, not a bad part. Worked if: Codes clear without replacing anything.
- Service the TAC connectors (dielectric grease, reseat) and verify TAC grounds. — Corroded or loose TAC connectors cause intermittent WOT limp mode that clears and returns. Worked if: No more limp mode on hard throttle after reseating.
How you know it's fixed
- P1125/P2120/P2125 stay cleared through multiple key cycles and full-throttle pulls.
- Scanner shows APP sensors 1 and 2 agreeing and the throttle blade following pedal position to 100%.
- No reduced-power message on hard acceleration.
Key numbers
| What | Value |
|---|---|
| Gen III DBW year-range compatibility | 2003-2005 share; 2005 pedal went 2-pot; 2006+ pedals not interchangeable |
| DBW mismatch fix requirement | matched pedal + TAC + throttle + matching tune |
| Why the PCM sets P1125/P2120/P2125 | APP/TPS disagreement → codes + reduced power |
Every option we know of
| Maker / product | Part # | Fits | Price | Where | Status / notes |
|---|---|---|---|---|---|
| GM (used/salvage) Matched pedal + TAC module + throttle body set | Must be a matched year-range set: pedal + TAC module + throttle body + matching PCM tune | Salvage yards, eBay | Discontinued / used only New units long discontinued; year mixing is the #1 cause of these codes |
Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.
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