For enthusiasts of efficient motoring, the Honda Civic Vx stands out as a beacon of fuel-sipping engineering. This special variant of the 5th Generation Honda Civic is celebrated for its unique engine, meticulously designed to maximize miles per gallon. Specifically, the 49-state (or Federal) version of the Civic VX came equipped from the factory with a sophisticated 5-wire wide band O2 sensor, also known as a Lean Air Fuel (LAF) sensor. Paired with a specialized Engine Control Unit (ECU), this system enabled the engine to operate in a lean burn mode. This resulted in impressive EPA fuel economy ratings of 39 mpg city, 50 mpg highway, and 43 mpg combined. These figures rival even modern hybrids, with the Civic VX achieving highway ratings that surpass even the Toyota Prius in its day!
However, stringent emissions standards in California presented a challenge. The lean burn technology, while boosting fuel efficiency, also produced slightly higher NOx emissions. Consequently, California-bound Honda Civic VX models were manufactured without the lean burn feature. These models, while retaining many of the Federal version’s attributes, were fitted with ECUs that deactivated lean burn and typically used a standard 4-wire O2 sensor. Despite these modifications, the California Civic VX still achieved commendable EPA ratings of 37 mpg city, 45 mpg highway, and 40 mpg combined.
For dedicated “ecomodders” and fuel efficiency enthusiasts, the allure of unlocking the Federal VX’s full potential is strong. Swapping to a Federal ECU is a well-known and relatively straightforward method to potentially boost fuel economy by 5 to 10% or even more. This guide aims to consolidate the scattered information available online into one comprehensive resource, providing a step-by-step walkthrough for converting a California Civic VX to Federal specifications and enabling lean burn for enhanced MPG. While this guide compiles existing knowledge and insights, it aims to provide clarity and incorporate practical experiences to assist fellow enthusiasts in this fuel-saving modification.
This guide prioritizes a completely reversible conversion approach. This philosophy ensures that the vehicle can be returned to its original California configuration, whether for emissions testing, resale purposes, or in case any issues arise during the conversion process. For those seeking a less reversible approach, modifying the stock wiring harness directly might be faster, but this guide focuses on a clean and reversible method.
Owners of California Civic VX models are particularly well-positioned for this conversion. Beyond the under-hood wiring necessary for connecting the wide band LAF sensor, the essential wiring within the main harness, linking the engine compartment to the OBD1 connector for the ECU, is already in place. This eliminates the need for complex firewall wire routing and OBD1 connector modifications, simplifying the “clean” conversion process significantly.
If you’re fortunate enough to own a ’92 California Civic VX model, there’s a possibility it might already have the 5-wire wide band LAF sensor pre-installed from the factory. In such cases, the conversion becomes remarkably simple: a mere ECU swap might be all that’s required to unlock lean burn capability!
Parts and Tools for Your Civic VX Conversion
To undertake this fuel efficiency upgrade for your Civic VX, you’ll need to gather the following components and tools:
Required Parts:
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Federal VX ECU: Acquire a used Federal VX ECU. Online marketplaces like eBay are excellent sources. Look for ECUs with the model number series followed by P07-A00 or P07-A01. Avoid ECUs with P07-L00 or P07-L01, as these are California models.
Alt text: Image of a Federal Honda Civic VX ECU label showing the P07-A00 model number, crucial for identifying the correct ECU for lean burn conversion.
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Wide Band LAF Sensor, Model L1H1: Obtain a new Honda factory Wide Band LAF sensor, model L1H1. While new sensors from dealerships can be expensive (around $400), keep an eye on eBay for potentially more affordable options, sometimes available for around $170.
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Connector Set: Source a set of connectors from a local auto salvage yard (Pick-N-Pull). Focus on Honda vehicles of similar vintage to your 5th generation Civic (1992-1995). You’ll need the following connectors:
- One 4-pin male connector (ensure pins #2 and #4 are present).
- One 8-pin female connector.
- One 6-pin male connector (ensure all 6 pins are present).
- One 6-pin female connector.
Important Connector Pin Notes: Honda connectors use a specific pin numbering system, as illustrated in the following images. Refer to these diagrams when wiring your harness.
Alt text: Diagram illustrating the pin numbering system for Sumitomo connectors used in Honda vehicles, essential for correctly wiring the Civic VX conversion harness.
Alt text: Image showing the pin numbering on 6-pin male and female connectors, crucial for accurate wiring during the Honda Civic VX lean burn conversion.
Salvage Yard Connector Harvesting Locations: Passenger shock tower areas often yield 6-pin connectors. Near the distributor and driver’s side shock tower are potential locations for 4-pin and 8-pin connectors.
Alt text: Photo highlighting potential locations in a salvage yard Honda engine bay where needed connectors for the Civic VX lean burn conversion can be harvested.
Alt text: Image showcasing a collection of harvested Honda connectors, the necessary components for building a reversible wiring harness for the Civic VX lean burn upgrade.
Required Tools:
- Soldering Iron, Solder, and Soldering Supplies
- Wire Strippers
- Electrical Tape
- Digital Multimeter (DMM) for continuity testing
- Civic VX Wiring Diagram (see below)
- Extra Wire
Building Your Civic VX Conversion Wiring Harness
Creating a custom wiring harness is key to a reversible and clean Civic VX lean burn conversion. The following wiring diagram and instructions, originally detailed by user monroe74 on GasSavers.org, will guide you through the process.
Wiring Diagram:
Alt text: Wiring diagram comparing 4-wire and 5-wire O2 sensor configurations for Honda Civic VX, illustrating the wiring changes needed for the lean burn conversion.
Wiring Instructions:
The 8-pin and 4-pin connectors will be positioned near the LAF sensor, while the 6-pin male and female connectors will be located near the passenger shock tower connectors. When preparing your wires, note that the four wires connecting the 8-pin male connector to the 6-pin male connector need to be approximately 2 feet long. The ground wire from the 8-pin male connector to the ground point should be around 1.5 feet (18 inches) long. Pre-measuring wires can streamline the soldering process.
Monroe74’s Wiring Instructions (GasSavers.org):
My goal is to connect the new parts (ECU and O2 sensor) without cutting any existing wiring. I want to do it all via standard Honda connectors, so the installation will be easily reversible, without a trace. Three ECU terminals (D3, D8 and D16) are used in the 49-state VX, but not in the CA VX. The good news is that the CA VX already has the needed wires in place in the main harness (at least this is true in my ’95 CA VX; I verified this with a continuity test). This means if I create a custom harness, using 4 connectors, I can install the new O2 sensor without cutting any wires.
I was able to find the needed connectors at a junkyard. I didn’t need to find a VX; the needed connectors are used in a bunch of different places on the other ’92-’95 Civics. The needed connectors are as follows:
8-pin female
6-pin male
6-pin female
4-pin maleYou create your custom harness by wiring them together as follows:
8-pin female.
- pin 1 needs to be wired to 4-pin male, pin 4.
- pin 2 needs to be wired to ground, G101.
- pin 3 needs to be wired to 4-pin male, pin 2.
- pin 4 needs to be wired to 6-pin male, pin 3.
- pin 5 is not used.
- pin 6 needs to be wired to 6-pin male, pin 2.
- pin 7 needs to be wired to 6-pin male, pin 6.
- pin 8 needs to be wired to 6-pin male, pin 5.
The wires from 4, 6, 7, and 8 need to be shielded, and the shield should be grounded.6-pin male.
- pins 2, 3, 5 and 6 are referenced above.
- pin 1 needs to be wired to 6-pin female, pin 1.
- pin 4 needs to be wired to 6-pin female, pin 4.
6-pin female.
- pins 1 and 4 are referenced above.
- pins 2, 3, 5 and 6 are not used.
4-pin male.
- pins 2 and 4 are referenced above.
- pins 1 and 3 are not used.
That’s a total of eight connections that have to be made, to join these 4 connectors together to form your custom harness. (There’s one more connection, a ground, that will be made when you’re installing the harness on the car.)
The harness is installed on the car as follows:
8-pin female. Connects to new O2 sensor, C111.
6-pin male. Connects on shock tower, to female of C129.
6-pin female. Connects to male of old C129.
4-pin male. Connects to old C111 4-pin female (the connector your old O2 sensor was plugged into).
Harness Assembly Workspace:
Alt text: Image of a workspace prepared for assembling the Honda Civic VX lean burn conversion wiring harness, showing connectors, wires, and soldering tools.
Begin by soldering wires to the 8-pin female connector, connecting them to the 4-pin male and 6-pin male connectors as per the instructions. Don’t forget to include the grounding wire. Next, solder the wires between the 6-pin male and 6-pin female connectors.
Crucial Step: Continuity Testing: After soldering, meticulously verify each connection with your Digital Multimeter (DMM) to ensure continuity and accuracy against the wiring instructions. Double-check pin numbers and male/female connector orientations to avoid errors.
Finished Harness Example:
Alt text: Image of a partially assembled Honda Civic VX lean burn conversion wiring harness, showing the soldered connector junctions before taping.
Once you’ve confirmed all connections are correct, use electrical tape to wrap the entire length of the harness. This protects the wires and provides a clean, professional finish.
Completed Wiring Harness:
Alt text: Image of a fully assembled and taped Honda Civic VX lean burn conversion wiring harness, ready for installation in the vehicle.
Installing the LAF Sensor and Wiring Harness
With your wiring harness built, the next step is installing the 5-wire LAF sensor and integrating the harness into your Civic VX.
LAF Sensor Installation:
Installing the 5-wire LAF sensor is generally straightforward. If you have an O2 sensor socket with a wire slot, use it. Otherwise, removing the exhaust heat shield will provide access to remove the original sensor with an open-end wrench. Apply a small amount of anti-seize compound to the new sensor’s threads before installation to facilitate future removal.
Harness Connection Points:
Connecting the harness is designed to be plug-and-play:
- The 8-pin female connector plugs directly into your new 5-wire LAF sensor.
- The 4-pin male connector plugs into the original connector where your old O2 sensor was connected.
- The 6-pin male and female connectors are inserted between the original 6-pin connectors near the passenger shock tower.
- The ground wire connects to the designated ground point (G101) on the engine block.
Harness Connected to LAF Sensor:
Alt text: Image showing the newly built wiring harness connected to the 5-wire LAF sensor in a Honda Civic VX engine bay, highlighting the clean integration.
Harness Connection Near Shock Tower:
Alt text: Image depicting the wiring harness connections near the passenger side shock tower of a Honda Civic VX, showing the integration with existing connectors.
Ground Point Connection (G101):
Alt text: Image illustrating the ground wire connection of the lean burn conversion harness to the G101 ground point on the Honda Civic VX engine block.
ECU Swap and Final Steps
The final piece of the conversion is replacing your California ECU with the Federal VX ECU.
ECU Location and Removal:
The ECU is located behind the passenger side kick panel inside the cabin. Accessing it involves pulling back the carpet to reveal the ECU connectors. Unfasten the connectors and carefully remove the ECU. Tilting the ECU bottom upwards and then rotating the top downwards can ease removal. Disconnect the three connectors to fully detach the California ECU.
ECU Location Behind Kick Panel:
Alt text: Image showing the location of the ECU behind the passenger side kick panel in a Honda Civic VX, indicating the access point for ECU removal and replacement.
Federal ECU Installation and Testing:
Connect the three connectors to your new Federal ECU. Before fully securing the new ECU, start your Civic VX and take it for a short test drive to confirm everything is functioning correctly.
Congratulations! If your Civic VX runs smoothly, you’ve successfully enabled lean burn! Reassemble the passenger compartment kick panel and enjoy the benefits of improved fuel economy from your converted Honda Civic VX.
