What Does P1789 Mean?
The transmission control module received an error message from the engine control module regarding idle speed intervention. This indicates a communication problem or functional issue between the two control modules during transmission events requiring idle adjustments.
Common Causes
35%
Communication network error between TCM and ECM
25%
Engine control module software issue or fault
20%
Transmission control module software issue or fault
12%
Idle air control system malfunction
8%
Damaged CAN bus wiring or poor module grounds
Diagnostic Steps
1
Step 1: Check for additional engine control codes that may affect idle control
2
Step 2: Verify communication between ECM and TCM using bi-directional scan tool
3
Step 3: Check CAN bus wiring and connections between control modules for damage
4
Step 4: Verify both ECM and TCM have proper power and ground connections
5
Step 5: Test idle air control system operation and verify idle speed is within specification
Estimated Repair Cost
$150 - $1,200
Parts + labor, varies by vehicle and location
The transmission control module received an error message from the engine control module regarding idle speed intervention. This indicates a communication problem or functional issue between the two control modules during transmission events requiring idle adjustments.
The most common cause of P1789 (Idle Speed Intervention Circ. Error Message from Engine Contr) is: Communication network error between TCM and ECM
Typical repair costs for P1789 range from $150 to $1,200, depending on the vehicle, location, and whether you do it yourself or go to a shop.
May cause rough idle, stalling during shifts, or poor shift quality. Vehicle is generally safe to drive but may experience driveability issues.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1789 to identify the root cause.
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Quick Info
Category
Powertrain
System
Transmission / Engine Controls
Difficulty
Type
Manufacturer
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