GM L5P emissions codes: rule out an exhaust leak. See the manufacturer document, vehicle conditions, exclusions and questions for a qualified shop before assuming a repair.
This is a research summary of a manufacturer communication, not a diagnosis for your car, a DIY repair procedure or confirmation of warranty or recall eligibility. No qualified technician has approved a repair for your individual vehicle here.
This historical communication is conditional. The current revision, VIN/build eligibility, measured diagnosis and any warranty or recall decision are not established here. Ask the manufacturer or a qualified shop to check the complete current document.
Read the General Motors original: L5P emissions DTCs and exhaust-leak diagnosis · Check open recalls by VIN at NHTSA Source date: 2024-08; research checked: 2026-11-01.
What prompted this manufacturer document
- An exhaust leak can contribute to these emissions faults, sometimes with an odor or hissing noise.
Which vehicle conditions must be checked
Each row below retains its own conditions. Do not combine engines, years or models across rows to create new applications. A listed model is not proof that a particular VIN is affected.
- Chevrolet Silverado · 2017, 2018 · L5P
- Chevrolet Silverado 2500HD / 3500HD · 2019, 2020, 2021, 2022, 2023, 2024, 2025 · L5P
- GMC Sierra · 2017, 2018 · L5P
- GMC Sierra 2500HD / 3500HD · 2022, 2023, 2024, 2025 · L5P
- Retain the source model-year table; GMC 2019-2021 is not silently filled into its gap.
- Smoke absence alone can be misleading because the filter can trap smoke.
Codes recorded in this document
A root listed only as an appearance has no separately verified meaning here. Preserve every suffix and module name; a shared root does not transfer a repair branch to another vehicle.
| Root | Raw form | Meaning in this source, if established |
|---|---|---|
| P0402 | P0402 | Appears in the document; individual meaning not verified |
| P0421 | P0421 | Catalyst-conversion performance monitoring fault in this GM application. |
| P11D5 | P11D5 | NOx sensor 2 learning-limit fault in this application. |
| P20EE | P20EE | NOx catalyst efficiency monitoring fault. |
| P2459 | P2459 | Particulate-filter regeneration frequency fault. |
| P2463 | P2463 | Particulate-filter soot-accumulation fault. |
| P249D | P249D | Reductant closed-loop control reached its low-flow limit. |
| P249E | P249E | Reductant closed-loop control reached its high-flow limit. |
What an owner can record safely
- Record odors, noises and all warning messages from outside the vehicle without touching hot exhaust.
What the source does not prove
- No improvised exhaust plugging, smoke generation or forced regeneration.
- A code or message alone does not establish a failed part, a safe-to-drive decision or an approved repair for this VIN.
- A service bulletin is not automatically a recall or free warranty repair. Confirm current eligibility directly.
Questions for the repair shop
- Was an exhaust leak excluded before condemning a NOx sensor, DPF or catalyst?
- Does a later revision or a current recall change the source's original advice?
- What measurement or inspection confirms the cause, and how will the repair be verified?
Read the original record
The linked communication may be revised after the research date. Compare its document number, exact vehicle table, diagnostic preconditions and exclusions with a qualified technician. This page paraphrases selected facts and does not reproduce the manufacturer procedure.
Sources and scope
- General Motors — L5P emissions DTCs and exhaust-leak diagnosis (23-NA-061) Document: 2024-08 · Research checked: 2026-11-01
Research snapshot: 2026-11-01. Source documents may have newer revisions. Verify current instructions and VIN eligibility with the manufacturer.