U1077 SCP Bus Primary ID Fault
Diagnostic trouble code U1077 indicates a failure in the SCP (Standard Corporate Protocol) communication bus, which is Ford's proprietary implementation of the J1850 PWM standard operating at 41.6 kbps. This code sets when a control module (ECU, PCM, or network module) detects that the primary identifier data is missing, corrupted, or invalid per SAE J1850 specifications. The primary ID is essential for module synchronization and critical data exchange on the vehicle's data bus.
1. Commonly Affected Makes
2. Most Common Causes
- Faulty master control module (PCM) that generates the primary ID
- Electromagnetic interference on the SCP bus (OBD-II pins 2 and 10)
- J1850 bus termination resistance out of spec (should be 120 ohms)
- SCP bus wiring shorted to ground, battery, or between lines
- Poor connections or corrosion at data bus connectors
- Defective control module not responding with a valid primary ID
- SCP bus voltage variations outside the nominal range (0-7V for J1850 PWM)
3. Symptoms You Will Notice
- Check Engine Light or other warning lights illuminated
- Communication loss between modules, causing drivability issues or no-start
- Intermittent or complete failure of systems like ABS, transmission, or instrument cluster
4. Step-by-Step Diagnosis
Scan all modules for related communication codes and verify U1077 is current
Inspect OBD-II connector pins 2 and 10 for damage, corrosion, or bent pins
Measure bus termination resistance between pins 2 and 10 (should be ~60 ohms with two 120-ohm terminators in parallel)
Check SCP bus wiring for shorts, opens, or interference using a digital multimeter and oscilloscope
Test voltage on SCP bus lines (idle voltage should be around 0V, active pulses 0-7V)
If wiring and termination are good, suspect a faulty module; isolate by disconnecting modules one at a time
5. Frequently Asked Questions
Is it safe to drive?▼
No, this is a high-severity fault. Driving may cause unpredictable behavior, loss of critical systems, or a no-start condition. Have the vehicle towed to a repair facility.
How much does it cost to fix?▼
Diagnosis typically costs $100-$200. Repair costs vary widely: simple wiring repairs $100-$300, module replacement $500-$1500+ depending on the module and labor.