Most automobiles display a check-engine light on the dashboard when an operating problem is detected by the on-board diagnostic system. A specific trouble code is then stored in the diagnostic system. The stored codes can be retrieved, and many of them are related to emissions. Vehicle owners receiving a code for insufficient exhaust gas recirculation (EGR) may need to clean or replace their EGR valve to pass a smog test.

You may have never noticed it, but a small tube is likely attached to the side of the exhaust pipe underneath your car. If you were to trace the path of the narrow tube, you would discover that it is routed back to the engine. The small passageway is referred to as the EGR tube, and its function is to recirculate a small amount of exhaust gas back into the engine.

EGR valve function

The recirculation of exhaust gas back to the engine slightly lowers the combustion temperature, thereby reducing the level of nitric oxide emissions. Exhaust gas recirculation is only needed under certain operating conditions, so an EGR valve is necessary. A check-engine code indicating insufficient EGR flow may be indicative that your EGR valve is either clogged or faulty.

EGR valve control

Before having the EGR valve serviced, take a few moments to check for a possible vacuum leak affecting the valve. The typical EGR valve contains a diaphragm that opens and closes in response to the vacuum supplied through a connected hose. Simply listen for an unusual hissing sound that might be indicative of a vacuum leak. If there are no signs of a loose vacuum hose, the EGR valve might need to be removed for inspection.

EGR valve servicing

Even though cleaning an EGR valve is relatively straightforward, gaining access to the valve may be difficult on some vehicles. If the valve is located toward the rear of the engine, additional components might need to be removed first to reach the valve. The repair of emissions components is regulated in some states, so you may need to have an experienced professional service the EGR valve.

Excessive nitric oxide is likely to result in a failed emissions test. Nitric oxide is only one of several gases measured during automobile smog tests. In addition to an exhaust tailpipe test, your vehicle’s on-board diagnostic system may be checked to ensure it is working properly and has no stored trouble codes. Contact a certified smog station like West Coast Smog for more information on emissions testing.