Ford Truck

2018-02-08 Check Engine Light with P21B0

• http://www.thedieselstop.com/forums/f149/p21b0-engine-code-542866/
• https://www.ford-trucks.com/forums/1271066-check-engine-light-code-p21b0-what-is-it.html
• Theory: CEL comes on when DEF tank is below half full

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