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I have a 1995 gmc Sierra hd with the 6.5L turbo Diesel engine in it. I just recently bought it and when ever I hold around 1300-1800 rpm the check engine light comes on, when I take my foot off the throttle and the rpms drop to 1000 the light goes out same with if it push the throttle to 2000 rpm the light will go out. I don't understand why this is happening if you can please help me out, thank you steven

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First thing comes to mind is a combustion problem.

It may be because of the Head Gasket which is a common issue for Sierra. Has your vehicle's oil recently been changed? And also has the gas cap changed? You should first check the oil cap and gas cap because the sensor gets triggerred by the smoke control which also causes the check engine light to blink. If this is the reason it may cause financial problems for you so you should better consider getting your car checked.

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