U1189: SCP Bus Communication Failure
Diagnostic trouble code U1189 indicates that a control module has detected invalid or missing data on the SCP (Standard Corporate Protocol) J1850 bus, specifically for the primary data identifier. SCP is a proprietary communication architecture developed by Ford Motor Company and used in their vehicles before the widespread adoption of CAN bus. This code means a module on the network is not receiving or transmitting the expected information on the primary data ID, which can disrupt coordination between modules such as the PCM, instrument cluster, or other controllers.
1. Commonly Affected Makes
2. Most Common Causes
- Faulty data transmitting module on the SCP bus
- Open or damaged wiring in the SCP bus circuits (BUS+ and BUS-)
- Incorrect or damaged bus termination resistor
- Receiving module with internal message processing failure
- Poor connections or corrosion in SCP bus connectors
- Electromagnetic interference affecting J1850 signal integrity
3. Symptoms You Will Notice
- Check Engine Light or other warning lights illuminated
- Erratic instrument cluster behavior or gauge failures
- Intermittent drivability issues such as stalling or hesitation
- Communication errors with scan tool or inability to communicate with certain modules
4. Step-by-Step Diagnosis
Scan all modules for related communication codes and note which modules are offline
Inspect SCP bus wiring for damage, corrosion, or loose connections at all module connectors
Measure resistance between BUS+ and BUS- at the data link connector (should be approximately 60 ohms with two termination resistors)
Check for proper termination resistors at the ends of the SCP bus (typically 120 ohms each)
Perform a voltage test on BUS+ and BUS- with ignition on (should be around 2.5V each, with a differential of 0V when idle)
Isolate the fault by disconnecting modules one at a time while monitoring bus activity
5. Frequently Asked Questions
Is it safe to drive?▼
The severity is medium. You may be able to drive short distances, but drivability issues or loss of critical functions (like instrument cluster) could occur. It's best to diagnose and repair promptly.
How much does it cost to fix?▼
Repair costs typically range from $100 to $500, depending on the cause. Simple wiring repairs may be under $200, while replacing a faulty module can cost $300–$500 or more.