U1120: SCP Bus Invalid Data
The U1120 code indicates that a control module has detected invalid or missing data on the SCP (Standard Corporate Protocol) J1850 communication bus, specifically for the primary data identifier. This protocol is a serial communication bus used by Ford Motor Company in older vehicles (roughly up to the mid-2000s). The code sets when a control module does not receive expected information from another module over the data bus, or when the received data is corrupt, out of range, or does not conform to the defined protocol format. The 'Primary Id' refers to the main message identifier that contains critical system information.
1. Commonly Affected Makes
2. Most Common Causes
- Faulty PCM or data transmitting module
- Electrical interference or noise on the SCP J1850 data bus
- Abnormal resistance in the SCP bus wires (SCP+ and SCP- circuits)
- Corroded, loose, or damaged connectors on the communication bus
- Faulty instrument cluster or module that transmits the primary identifier
- Weak battery or charging system with irregular voltage
- Control module with corrupt or outdated internal software
3. Symptoms You Will Notice
- Check Engine Light or other warning lights on
- Erratic gauge readings or non-functional gauges
- Drivability issues such as stalling, hesitation, or poor performance
- Communication failures between modules (e.g., no response from PCM)
4. Step-by-Step Diagnosis
Scan for all stored codes and freeze frame data
Inspect battery voltage and charging system; ensure stable 12.6V+
Visually check SCP bus connectors for corrosion, damage, or looseness
Measure resistance between SCP+ and SCP- at the DLC; should be approximately 60 ohms
Check for electrical noise or interference on the bus with an oscilloscope
Test individual modules by disconnecting them one at a time to isolate the fault
If necessary, update or reprogram the affected module's software
5. Frequently Asked Questions
Is it safe to drive?▼
The severity is medium. You may be able to drive, but expect drivability issues and potential communication failures. It's best to diagnose and repair promptly to avoid further problems.
How much does it cost to fix?▼
Costs vary widely depending on the root cause. Simple fixes like cleaning connectors or replacing a battery can be under $100. Module replacement or reprogramming can range from $200 to $800 or more.