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Q: I took my car to my mechanic for the check engine light being on. He checked the codes and said it’s an “EGR fault” He said it would be a couple of days before he could get to it. He said I could keep the car but shouldn’t drive very far until he fixed it. Can you explain what this is about? Will I hurt it if I drive it?

— Ann T.

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A: Hmmm … perhaps his bedside manner could use a little work! Another moment or two of conversation would have put you at ease.

Most current engines employ exhaust gas recirculation, or EGR, to reduce combustion temperature in order to mitigate the formation of oxides of nitrogen, a nasty pollutant. A small quantity of exhaust (inert) gas is combined with the incoming combustion air to take up space that would have been air and fuel. This is usually done via an electric solenoid or vacuum servo and solenoid (older) that receives signals from the powertrain control module. Various means of confirming EGR function are used so that faults can be readily identified and corrected. Occasional issues may occur due to the EGR devices failing, or perhaps clogging of the EGR passages in the cylinder heads.

Engines actually run well without EGR, but with a higher than normal combustion temperature there’s a chance of preignition or detonation. Preignition (a jingling sound may be heard) is a slight to moderately harmful form of abnormal combustion, a collision of accidentally and intentionally ignited combustion flame fronts. Detonation (a knocking sound) is a very harmful explosion of combustion gasses, rather than a rapid burn, that will likely damage engine parts.

Preignition and detonation are more likely to occur during hot weather, under demanding engine conditions, or if lower than specified octane fuel is used. Colder weather, gentle low speed diving and perhaps a tank of premium grade fuel should reduce the chances of problems as a short term fix, should EGR repairs need to be delayed. Many vehicles also employ a detonation sensor so that corrective adjustments to the engine can be automatically made to reduce or eliminate both preignition and detonation.

It’s tough to say which of several possible EGR fault codes was seen by your tech. Perhaps it was a P0401 (insufficient EGR flow)? This could result in minor symptoms and be perhaps a somewhat time consuming process to roto-rooter hard carbon from stubbornly clogged passages. Other P0400-P0408 codes may be set due to an inoperative EGR valve, sensor or possible wiring fault. These are likely quicker/easier to fix but may result in no EGR function.

I think your tech was trying to play it safe with his recommendation. With cold weather, some easy/short trips as needed shouldn’t be a problem. If you should hear the above mentioned noises as you accelerate, of course avoid driving further, other than heading for home or the shop.

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