GMC Code P118B: Meaning, Causes & How to Fix It
Code P118B is specific to diesel-equipped GMC models and relates to the particulate matter sensor — a component that monitors the exhaust particulate filter to confirm it's properly trapping emissions. This code means the sensor's temperature reading doesn't make sense to the computer, rather than reflecting an actual emissions failure by itself.
Jump to a section: What P118B means · Common symptoms · Common causes · How it's diagnosed and fixed · Quick reference table · FAQ
What P118B Means
The particulate matter sensor sits in the exhaust system and monitors the diesel particulate filter's effectiveness by measuring the amount of soot passing through. When the sensor's reported temperature doesn't fall within a range the computer considers plausible, it logs P118B, treating the reading as unreliable rather than acting on it.
Common Symptoms
Poor engine performance, decreased fuel efficiency, and an illuminated check engine light are the most commonly reported symptoms, though many owners notice no dramatic drivability change at all beyond the warning light itself.
Common Causes
A malfunctioning particulate matter sensor, damaged or corroded sensor wiring, a loose or poor connector, or a broader electrical circuit fault affecting the sensor's readings are the most frequently reported causes.
How It's Diagnosed and Fixed
Visual inspection of the sensor and its wiring for damage or corrosion is the logical starting point, followed by a sensor test with a proper scan tool to verify actual functionality and reading accuracy. If wiring and connections check out fine, sensor replacement is the standard fix.
Quick Reference Table
| Step | What To Check |
|---|---|
| 1 | Inspect particulate sensor and wiring visually |
| 2 | Check connector for corrosion or looseness |
| 3 | Test sensor functionality with a scan tool |
| 4 | Replace the sensor if it fails testing |
Frequently Asked Questions
Is P118B urgent to fix?
It's not typically an emergency, since core drivability usually isn't dramatically affected, but it's worth addressing to restore accurate emissions monitoring and avoid a potential emissions test failure.
Will this code affect my diesel particulate filter's actual function?
Not directly — it reflects a sensor reading problem rather than an actual filter malfunction, though it does mean the system can't properly verify filter performance until the sensor issue is resolved.
Can I clean the sensor instead of replacing it?
Sometimes a sensor affected by excessive soot buildup can be cleaned and tested again, though a sensor with an internal electrical fault will need replacement regardless of cleaning.
Does this code only appear on diesel models?
Yes — the particulate matter sensor is specific to diesel exhaust aftertreatment systems, so this code won't appear on gasoline-powered GMC models.
Shop the Parts Mentioned
- Bidirectional Scanner — tests particulate sensor functionality directly.
- Wireless Scanner — checks for related emissions and sensor codes.
- Taylor Battery Cable Terminal Kits — addresses related wiring and connection issues.
- 95-Piece Mechanics Tool Set (Metric & SAE) — for sensor and exhaust system access.
- 5-Piece Pro Electric Wrench Kit — speeds up exhaust component access.