Symptoms
- Emissions test shows NOx above the limit
- Possible pinging or knocking sound under load
- Engine may run slightly hot
- Check engine light possibly on with an EGR-related code
Likely causes
- CommonStuck or clogged EGR valveThe EGR system lowers combustion temperature; when it's stuck closed, NOx rises.
- CommonCarbon-clogged EGR passagesCarbon buildup blocks the exhaust gas recirculation path even with a working valve.
- PossibleCooling system running the engine hotHigher combustion temperatures increase NOx formation.
- PossibleLean air/fuel mixtureA vacuum leak or sensor issue can push the mixture lean, raising combustion temperature.
- PossibleFailing catalytic converter (three-way function)An aged converter loses some ability to reduce NOx as well as HC and CO.
What to do
- Get the check engine light scanned for EGR-related or lean-condition codes.
- Have the EGR valve and its passages inspected for carbon buildup or a stuck position.
- Check the cooling system: coolant level, thermostat operation, and radiator condition.
- Have fuel trims checked for a lean condition and any vacuum leaks repaired.
- Clean or replace the EGR valve if it's found to be stuck or clogged.
- Retest at an emissions station once repairs are complete and any check engine light has cleared.
Tools and parts
- OBD-II code reader
EGR valveSearch: “EGR valve for [year make model]”
ThermostatSearch: “engine thermostat for [year make model]”
When to call a pro
A shop can test EGR flow and cooling system temperature directly, which is hard to diagnose by feel alone.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.