When
glow plugs become weak, the engine will take longer than 3-5 seconds to start, and may not start at all after they degrade. Diesel engines with external block heaters have a better chance of starting the diesel engine than those without. The glow plugs can also cause the engine to run rough until it reaches near-operating temperature, because one cylinder will misfire. Smoke will billow from the tailpipe in this case, as raw fuel is dumped into, and burned in the exhaust. The check engine light will illuminate, and OBD codes will suggest the fuel delivery system, emissions and exhaust system, and possibly glow plugs. When the engine is warm, and restarted, the check engine light may disappear.