OBD-II read ERR from my 1998 Dodge Grand Caravan 3.8

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I have the shop mechanic checked it out and they said it would cost $700 to replace the ECM. I have the check engine light on, speedometer and odometer is not working so as the cruise control. But the 1998 Dodge caravan run fine, except I cannot read the diagnostic from OBE2, it would says ERR. So I cant get smoke check to register. When I want to order the ECM and replace it myself, Autozone told me ECM may not be the problem if the engine run fine, I should get a second opinion? or should I change the ECM? Is it the ECM that I could not read with the OBD2? I have the shop mechanic checked it out and they said it would cost $700 to replace the ECM. I have the check engine light on, speedometer and odometer is not working so as the cruise control. But the 1998 Dodge caravan run fine, except I cannot read the diagnostic from OBE2, it would says ERR. So I cant get smoke check to register. When I want to order the ECM and replace it myself, Autozone told me ECM may not be the problem if the engine run fine, I should get a second opinion? or should I change the ECM? Is it the ECM that I could not read with the OBD2?