OBD2 Code U0100: Lost Communication with ECM/PCM
Reviewed by ASE Certified Mechanics · Last updated September 01, 2026
Quick Answer
U0100 = Lost Communication with ECM/PCM
Severity: Check Soon · System: Network · ⛔ Do NOT continue driving
⚠️ STOP DRIVING — This code indicates a critical safety issue. Continuing to operate the vehicle may cause engine damage or safety hazard. Pull over safely and diagnose before continuing.
The U0100 code means Lost Communication with ECM/PCM. Estimated repair cost: $200 – $1500.
📋 Diagnostic Reference
| OBD2 Code | U0100 |
| Category | Network |
| Severity | Check Soon |
| Repair Cost | $200 – $1500 |
| Can You Drive? | ⛔ STOP NOW if flashing / ⚠️ Diagnose soon if steady |
🔍 Symptoms
- Check Engine Light on
- multiple warning lights
- engine may not start or run rough
- transmission may default to limp mode
⚠️ Most Likely Causes (in order of probability)
- Failed ECM/PCM
- damaged CAN bus wiring
- loose ECM connector
- low battery voltage
- water damage to ECM
🛠️ Diagnostic Procedure
Step 1: Check battery voltage — low voltage causes communication errors
Check battery voltage — low voltage causes communication errors
Step 2: Inspect ECM connectors for corrosion or water
Inspect ECM connectors for corrosion or water
Step 3: Check CAN bus wiring for damage
Check CAN bus wiring for damage
Step 4: Scan all modules — if only one module lost communication the CAN bus is likely...
Scan all modules — if only one module lost communication the CAN bus is likely fine.
🔧 DIY Diagnosis: Step-by-Step
Follow this diagnostic flow for U0100 (Network Communication & Data Link Failures). Start at Step 1 and work your way down — the most likely fixes are listed first.
- Check the battery and charging system — low voltage is the most common cause of communication loss codes
- Inspect the OBD2 diagnostic port for damaged pins or corrosion — a bad port affects all module communication
- Check CAN bus wiring for damage — look for chafed or broken wires in the main harness
- Verify all module ground connections are clean and tight — poor grounds cause communication failures
- If multiple modules are offline, check the CAN bus terminating resistors (should read ~60 ohms across pins 6 and 14 at the OBD port)
- Try disconnecting modules one at a time — a single failed module can pull down the entire CAN bus
📊 Common Causes & Repair Costs
For U0100, here are the most likely root causes ranked by how often mechanics see them, plus typical repair costs for each.
| Rank | Cause | Frequency | Typical Cost |
|---|---|---|---|
| #1 | Low battery voltage or voltage spike |
30%
|
— |
| #2 | CAN bus wiring damage or corrosion |
25%
|
— |
| #3 | Failed module pulling down the bus |
20%
|
— |
| #4 | Poor ground connection |
15%
|
— |
| #5 | OBD2 port damage or corroded pins |
10%
|
— |
Category Repair Cost Range (Network Communication & Data Link Failures)
| DIY / Best Case: | $50 |
| Independent Shop: | $400 |
| Dealer / Worst Case: | $2000 |
Costs include parts + labor. Actual price varies by make, model, and your location.
🚨 Address Immediately
Can you drive? Limited — Communication loss means the ECM can't talk to other modules. The car may run but features like traction control, ABS, or transmission shifting may be affected. Drive carefully to a shop.
Estimated Repair Cost
Typical repair: $200–$1500 (Parts + Labor) · Costs vary by make/model and location
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❓ Frequently Asked Questions
Can I drive with a U0100 lost communication code?
The engine may still run, but critical systems (ABS, traction control, transmission) may be affected. Drive very carefully and directly to a repair shop. Do not drive on highways until fixed.
How much does it cost to fix a U0xxx code?
Battery or ground repair: $50–$200. CAN bus wiring repair: $150–$500. Module replacement: $500–$2,000. Simple fuse or connector fix: under $50. Always start with the cheapest possibilities.
What does 'Lost Communication with ECM/PCM' mean?
It means the TCM or another module can't communicate with the engine computer over the CAN bus. This could be a wiring issue, a bad module, or low voltage — not necessarily a bad ECM.
Can a bad battery cause a U0100 code?
Very commonly. When battery voltage drops below ~10V, modules start dropping off the CAN bus. A dying battery or bad alternator can trigger U-codes. Always test the charging system first.
How do I test the CAN bus at home?
With the ignition off, measure resistance between pins 6 and 14 at the OBD2 port. You should read approximately 60 ohms (two 120-ohm terminating resistors in parallel). If you read 120 ohms, one resistor is missing. If you read 0 or infinite, there's a short or open circuit.