U0115 Code: Lost Communication With ECM/PCM "B"
Modules lost contact with the second engine computer on vehicles that use two of them.
Unverified: this manufacturer-specific definition comes from a single source or varies by model. Confirm it with a brand-specific scan tool.
Stop driving
If the engine runs rough, has lost significant power or stalls, don't drive it; a shut-down bank loads the catalytic converters with unburned fuel. If it runs normally with only a stored code, drive gently to a shop soon.
At a glance
Severity: High
- Meaning
- Lost Communication With ECM/PCM "B"
- System
- Module Communication · Network
- Safe to drive?
- With caution
- Typical repair cost
- $20–$2,500
- DIY difficulty
- Hard
Some engines are run by two engine computers instead of one: the main module (A) and a second module (B). Typical examples are large V10 and V12 engines with one computer per cylinder bank, and certain heavy-duty or diesel applications where a second module handles part of the engine or aftertreatment control. U0115 means the main ECM or another module stopped receiving messages from ECM B. What 'B' controls, and so what happens when it goes silent, differs by manufacturer, which is why this page is marked unverified: always check the service information for your vehicle. On a two-bank setup the usual result is one bank shut down, a rough engine and heavily reduced power; on other designs it can mean a no-start.
How to read U0115
- U Network
- 0 Generic (SAE)
- 1 Module Communication
- 15 Specific fault
Symptoms
- Engine runs rough, as if half the cylinders are dead
- Major loss of power and reduced-power warning
- Check engine light, often with misfire or fuel-trim codes on one bank
- Hard start or no-start on some designs
- Scan tool sees ECM A but can't open ECM B
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Blown fuse or failed main relay feeding ECM B (it often has its own supply)
- Corroded, water-soaked or loose connector at ECM B
- Damaged dedicated data link between ECM A and ECM B
- Low system voltage or poor engine/body ground
- Internal ECM B failure
- Software or coding mismatch after one module was replaced
Diagnosis step by step
Identify ECM B for your vehicle
Look up which module the manufacturer calls ECM/PCM B, where it's mounted and whether it shares or has separate fuses and relay. On two-bank engines both modules often sit side by side in an electronics box.
Tools: OBD-II scanner
Compare the two modules on the scan tool
Try to connect to both modules. If ECM A answers and ECM B never does, focus on B's supply and link; if both drop in and out together, look at shared power, grounds or the main network.
Tools: OBD-II scanner
Test ECM B fuses, relay and grounds
Measure each fuse with a meter and check that the main relay actually delivers battery voltage to ECM B when the key is on. Do a voltage-drop test on its grounds while cranking; above about 0.1 V points to a bad ground strap or splice.
Tools: Multimeter
Inspect connectors and the electronics box
Open the module box or cover and look for water, corrosion, a clogged drain or a missing seal. Unplug ECM B and check for pushed-out or green pins.
Tools: Flashlight, Screwdriver
Check the link between A and B
Many two-computer engines use a private data link between the modules. With the key on, check its bias voltages at both ends; with the battery off, check continuity and resistance per the service manual, including any terminating resistor.
Tools: Multimeter
Replace and code if proven faulty
Only replace ECM B after power, grounds and the data link test good. A new or used module must be programmed and matched to ECM A and the immobilizer.
Tools: OBD-II scanner
Where the parts are
- 1Battery
- 2Alternator
- 3Engine computer (ECM)
- 4CAN bus
- 5Transmission
- 6Wheel speed sensor
- 7High-voltage battery
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace fuse or main relay for ECM B | Yes | $20–$150 |
| Repair connector, drain or module-box seal | No | $150–$500 |
| Repair data link wiring between modules | No | $200–$600 |
| Replace and program ECM B | No | $900–$2,500 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Depends
If U0115 turns on the check engine light, which is typical when one bank or function drops out, the vehicle fails a US emissions test; the station may also be unable to read full readiness data.
Frequently asked questions
Why would a car have two engine computers?
Big multi-cylinder engines need a lot of injector and ignition outputs, so some makers split the work between two modules, often one per bank. Other designs use a second module for specific functions.
What's the difference between U0100 and U0115?
U0100 refers to the main engine computer (A) and U0115 to the second one (B). On a single-computer car you normally won't see U0115 at all.
Is it safe to drive on one bank?
Not for long. The engine is unbalanced and weak, and unburned fuel can overheat the catalytic converters. Get it to a shop gently or have it towed.
Why is this page marked as unverified?
The SAE name is fixed, but which module is 'B' and what it controls varies between manufacturers. Always confirm with the service information for the specific vehicle.
Sources
- SAE J2012 – Diagnostic Trouble Code Definitions
- ISO 15031-6 – Road vehicles, diagnostic trouble code definitions (EOBD)
- Manufacturer public service information (cross-checked)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy