P2668: Fuel Mode Indicator Lamp Control Circuit
What to do
The car drives normally and the fuel system is working — only the indicator has failed, so you are not being told which fuel you are on.
The main practical risk is running a tank dry without warning, since you cannot see which supply is in use or how much is left in it. Until it is fixed, keep both fuels topped up rather than relying on the display.
Worth checking your fuel costs while it is out of action. If the vehicle has quietly reverted to petrol you will be paying considerably more per mile with nothing on the dashboard to say so, and that is often the first sign anyone notices.
On an aftermarket gas installation the changeover switch and its indicator are usually one unit, so this often turns out to be that assembly rather than anything in the vehicle's own wiring.
What this code means
A fault in the circuit driving the lamp that tells the driver which fuel the engine is currently running on.
On a bi-fuel vehicle that lamp is doing more work than an indicator light usually does, which is why the standard gives it a code of its own. An engine that can run on petrol or on gas switches between them automatically — on start-up, when the gas tank empties, when a fault appears, and sometimes under load. The driver has no other way to know which fuel is being consumed, because the engine sounds and behaves near enough identically on both.
The practical consequences of losing that information are ordinary but real: a gauge you cannot trust means running a tank empty without warning, and a vehicle that has quietly reverted to petrol will report fuel consumption and cost that make no sense until somebody notices. On a vehicle bought for the running-cost saving, that is the saving disappearing invisibly.
Nothing about the engine is affected. The lamp is an output; the fuel system carries on switching correctly whether or not anybody is told about it. This is a low-severity code in the honest sense — the vehicle is entirely sound and a piece of information has gone missing.
Causes are the ordinary ones for a lamp circuit: the bulb or its driver, the wiring, the connector, or the instrument cluster. On an aftermarket installation the changeover switch and its indicator are frequently one assembly, so a failure often takes the switch's own display with it.
Likely causes
Start here
- Open circuit in module wiring · moderate
- Corroded or loose connector · easy
Less common
- Failed alternative fuel control module · specialist
- Wiring shorted to ground · moderate
- Control module driver failure · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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