U1118 SCP Bus Invalid Primary ID
The U1118 code indicates that a control module has detected invalid or missing data for the primary identifier (Primary ID) on the SCP (Standard Corporate Protocol) communication bus, which is based on SAE J1850. This serial network protocol was used by Ford Motor Company for data exchange between electronic modules such as the PCM, GEM, and ICM. The Primary ID is a priority message on the bus; when it is corrupted or not received, the diagnosing module logs a communication fault. This DTC specifically points to a problem in the data link layer, not a direct mechanical drivetrain defect.
1. Commonly Affected Makes
2. Most Common Causes
- Internal failure or communication lock-up in the control module responsible for transmitting the Primary ID
- SCP bus wiring shorted to ground, reference voltage, or open circuit
- SCP bus termination resistors out of specification, damaged, or disconnected
- Data bus connectors with retracted pins, corrosion, moisture, or poor retention
- Electromagnetic interference degrading signal integrity on the SCP bus
- Irregular battery voltage or weak charging system causing module resets or errors
- Faulty receiving module that incorrectly processes valid bus messages
3. Symptoms You Will Notice
- Check Engine Light or other warning lights illuminated
- Intermittent or complete loss of communication with certain modules, leading to gauge or accessory failures
4. Step-by-Step Diagnosis
Scan all modules for related communication codes and verify U1118 is current
Inspect SCP bus wiring and connectors for damage, corrosion, or loose pins
Measure SCP bus voltage and resistance at the data link connector; compare to specifications
Check termination resistors for proper resistance (typically 120 ohms each)
Perform a network integrity test to isolate the faulty module or wiring issue
5. Frequently Asked Questions
Is it safe to drive?▼
The vehicle may still be drivable, but communication issues can cause erratic behavior or loss of critical functions. It is recommended to diagnose and repair promptly to avoid being stranded.
How much does it cost to fix?▼
Repair costs vary widely depending on the root cause, ranging from $50 for a loose connector to $500 or more for a faulty module or extensive wiring repair.