Here are some possibilities to investigate, starting with the most likely:
* Transmission Problems: Transmission issues are a much more common cause of both a check engine light (the transmission sends codes to the ECM) and shifting problems. The problem could be:
* Low Transmission Fluid: Check the transmission fluid level and condition. Low fluid or dirty fluid can cause significant shifting problems.
* Worn Transmission Components: Clutches, solenoids, valve body, or other internal components may be worn out, especially given the age of the car. This is a significant repair.
* Transmission Sensor: A faulty sensor (like a speed sensor or shift solenoid) can cause erratic shifting and illuminate the check engine light. These are relatively inexpensive to replace compared to a full transmission overhaul.
* Wiring Harness Issues: A damaged or corroded wiring harness connecting the transmission to the ECM could be intermittently causing communication problems, mimicking ECM failure. This is particularly likely given the age of the vehicle.
* Other Sensors: Faulty sensors (like the crankshaft position sensor, throttle position sensor, or mass airflow sensor) can send incorrect data to the ECM, affecting the engine's performance and potentially causing transmission shifting problems. These sensors are relatively inexpensive and easy to replace.
* Power Supply Issues: Intermittent power to the ECM, perhaps from a bad battery connection or alternator, could cause unpredictable behavior. Get your battery and charging system tested.
What to Do:
1. Get the Diagnostic Trouble Codes (DTCs): This is crucial. Use an OBD-II scanner (available at auto parts stores) to read the codes stored by the ECM. These codes will pinpoint the likely problem area. Don't just erase the codes; write them down.
2. Check the Transmission Fluid: This is the quickest and cheapest thing to check.
3. Consult a Repair Manual: A repair manual specific to your 1999 Dodge Intrepid will give you detailed information on diagnosing and troubleshooting transmission and engine problems.
4. Seek Professional Help: If you're not comfortable working on your car yourself, take it to a qualified mechanic specializing in transmissions. Explain the history of the problem and the DTCs you obtained. They can diagnose the issue more accurately and avoid unnecessary repairs.
Replacing the ECM again without addressing the root cause is likely to be a waste of money and time. Focus on finding the *underlying* problem causing the symptoms, not just the symptom itself.
