U0118

U0118 Code Symptoms, Causes, Diagnosis & Repair Steps Manual

Quick Summary

U0118 means the PCM cannot talk to the Fuel Additive Control Module (FACM).
Drivers usually see the check‑engine light, a drop in fuel‑economy, or occasional rough‑idle/hesitation.
Causes are wiring/connectors, voltage spikes, or a failed FACM that has lost internal communication.
Diagnosis requires a bi‑directional scan, continuity testing, and, if needed, re‑flashing or replacement of the FACM.
Replacement units are VIN‑matched, pre‑programmed and installed plug‑and‑drive; Flagship One supplies these modules with a warranty.

Introduction

When the check‑engine light illuminates and you notice a subtle loss of fuel efficiency, the problem may lie in a communication breakdown between the powertrain control module and the Fuel Additive Control Module (FACM). The U0118 diagnostic trouble code (DTC) appears when the PCM reports that it has lost contact with the FACM, the component that meters additive dosing into the fuel stream. Because the FACM supplies data used for emissions calculations and fuel‑system timing, its silence can trigger the engine‑monitor light and cause the engine to run slightly leaner, resulting in reduced miles per gallon and occasional rough‑idle or hesitation. Early identification of the loss‑of‑communication condition prevents unnecessary wear on other control units and keeps the vehicle within emissions compliance.

DTC U0118 · engine computer (ECM/PCM)

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Symptoms

Check‑engine illumination – the MIL (malfunction indicator lamp) flashes or stays solid as soon as the PCM detects the loss of FACM data.
Reduced fuel‑economy – owners typically report a 2‑5 % drop in miles per gallon after the code appears.
Occasional rough idle or hesitation – the engine may stumble at idle or during light throttle because the PCM is operating without additive‑dose feedback.
Emissions‑related readiness loss – the vehicle may fail an OBD‑II emissions test until the code is cleared and the FACM communication is restored.

Why Fuel Additive Control Module Problems Occur

Faulty Wiring or Connectors

The FACM communicates with the PCM via a dedicated CAN‑high/low pair or a proprietary serial line. Corrosion, chafed harnesses, or loose pins break the signal path, causing the PCM to time‑out and set U0118.

Voltage Spikes or Ground Issues

Transient voltage spikes from the alternator or a weak battery ground can corrupt the FACM’s internal processor. Repeated spikes may fry the module’s communication transceiver, resulting in intermittent or permanent loss of contact.

Internal FACM Failure

The FACM contains a microcontroller, memory, and dosing pump driver circuitry. Water intrusion, solder‑joint fatigue, or component aging can cause the module to stop transmitting its status frames. In many cases the FACM will still power up, but the communication controller is dead.

Software Corruption

Incorrect re‑programming of the FACM or a failed OTA update can leave the module’s firmware in an unrecoverable state. The PCM then receives no valid data and logs U0118.

Diagnostic and Repair Procedures

Retrieve the DTC – Connect a bi‑directional scan tool and confirm U0118. Record any additional codes that may indicate related network faults (e.g., U0100, U0140).
Inspect wiring harness – Visually examine the FACM connector for corrosion, bent pins, or damaged insulation. Use a multimeter to verify continuity on each CAN or serial line and check for proper ground resistance (< 5 Ω).
Perform a communication test – Many scan tools have a “Network Scan” function that shows real‑time traffic. Absence of FACM frames confirms loss of communication.
Check power supply – Measure voltage at the FACM power pin while the ignition is on; it should be within 11.5‑14.5 V. Low voltage suggests a grounding or alternator issue, which should be addressed before proceeding.
Re‑flash the FACM – If the module’s firmware is suspected, use the manufacturer’s re‑programming procedure. This often requires a dealer‑level tool; a qualified independent shop can perform it with the proper software.
Module repair vs. replacement decision – If the communication test shows no signal after wiring and power checks, and re‑flashing does not restore traffic, the FACM is likely internally damaged.
Replace the FACM – Obtain a VIN‑matched replacement, have it programmed to the vehicle’s software version, and install it plug‑and‑drive. Clear the code and verify that the check‑engine light stays off and fuel‑economy returns to normal.

Typical costs – Re‑flashing may run $150‑$250 in labor. A VIN‑matched FACM replacement, including programming, generally costs $600‑$900 plus $150‑$250 labor.

When Fuel Additive Control Module Replacement Makes More Sense Than Repair

Repair of a FACM is feasible only when the failure is limited to a single component such as a blown fuse or a damaged connector. Once the internal communication controller or microprocessor is compromised, repair becomes a temporary fix; the module may fail again after a short drive cycle.

Modern control modules are deeply integrated with security, immobilizer, and emissions systems. That integration means a replacement must be correctly programmed to the vehicle’s VIN and software revision before it can communicate on the network. Using an OEM‑spec replacement that is not VIN‑matched can trigger additional DTCs or cause the vehicle to remain in a limp mode.

Flagship One specializes in VIN‑matched control modules, providing a plug‑and‑drive solution backed by a warranty. Replacement units are pre‑programmed to the exact specifications of the vehicle, eliminating the need for on‑site coding and reducing installation time. The warranty covers both the hardware and the programming, giving owners confidence that the new FACM will remain functional for the life of the vehicle.

When the cost of a professional repair approaches or exceeds the price of a new, VIN‑matched module, or when the vehicle is more than five years old and the original FACM is out of warranty, replacement is the more reliable and cost‑effective path.

Preventive Maintenance

Inspect connectors every 30 000 mi – Look for moisture, corrosion, or loose pins. A quick spray of dielectric grease can protect against future corrosion.
Maintain a healthy charging system – Keep the battery voltage above 12.4 V at rest and ensure the alternator output stays within spec (13.8‑14.8 V). Stable voltage reduces the risk of spikes that can damage the FACM.
Avoid fuel additives not approved by the manufacturer – Unapproved additives can introduce contaminants that accelerate internal FACM wear.
Run a full system scan at major service intervals – A scan that includes network health checks can catch early communication degradation before the U0118 code is set.