Fuel filter light; what should you do if it comes on?

If you are driving a vehicle along, and you notice a light comes up on the dash that you aren’t familiar with, you should pull over when safe and investigate. Your owners manual is generally the first place to start, and one of the more common items that can pop up is your fuel filter light.

Believe it or not, but many 4WD log book services do not cover the fuel filter. Isuzu do not change their factory fuel filters until the light comes on, and that’s pretty poor in my opinion.

Regardless, you should be running clean, quality diesel fuel through your engine and that means ensuring your fuel filtration system is in good condition.

Reliable 4WD
If you get a fuel filter light illuminate, what should you do?

What fuel filter lights are there?

In general, you’ll have two fuel filter lights that come on. One is in relation to water in the diesel, which is very, very bad, and can destroy your motor if you don’t get onto it quickly.

I’d pull over immediately if you see this, and turn the engine off. Check for water, and treat it as a pretty serious mechanical issue.

Water going into your motor can cause immensely expensive problems (like a full motor replacement), and that’s without the tow to get you to a mechanic!

The other one is related to flow, and comes on when the engine cannot get the flow of fuel that it wants. This is generally from a blocked filter, and if you have an older diesel instead of a light on the dash, you’ll just get no power at all.

We had a 2 micron filter (no idea why, that’s what the previous owner put in) on our 1HDT 80 Series, and the filter blocked up on the way to Jurien Bay (presumably algae) and it would just die at the worst possible times, limiting your speed to what ever fuel you could get through.

A quick carpark stop and a mate who’s a guru with engines and we had a new filter (always carry a spare) installed and it was good as gold.

30 micron pre filter
We always carry a spare primary, and secondary fuel filter now

The fuel filter light comes and goes

If your fuel filter light comes and goes, its due to a lack of flow, and it will almost always come on when you have your foot down a lot.

As soon as you back off the throttle the fuel filter light will go away. You are generally OK to keep driving like this and just get the filter replaced as soon as possible, but keep an eye on how often it comes on.

Draining water from your diesel fuel filter

Most diesel fuel filters have a tap at the bottom which you can undo and drain water from, or they might have a bowl that you can remove.

Either way, letting water go through your fuel filter and into the engine is not something you want to mess with!

Draining water from fuel
Our Isuzu manual shows how to drain water out of the OEM fuel filter

Do you know what to do if a fuel filter light comes on? If it goes away, I’d probably keep driving, but if it stays on you should pull up ASAP and make sure there’s no water in the filter!

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  1. Hey Brian,

    I haven’t seen anything here, but I’d be surprised if you couldn’t get an aftermarket solution. I’d question if you really need it though; why wouldn’t you just carry a spare filter and if the factory alarm comes up for it being blocked, stop and swap it if needed, or get your mechanic to do it?

    On the Dmax, it has a flow restriction alarm, which you see come up when you give it a bootful (up a hill etc), and it goes off again when you are just cruising along and using minimal fuel.

    This is enough to allow you to continue and get it changed soon, but not immediately.

    You could pick up one bad batch of fuel and get the filter blocked. Does knowing in advance offer any benefit?

    All the best

  2. Brian Symons says:

    I’ve seen comments about aftermarket Diesel Fuel Filter Alerts overseas that warn you as the filter is getting clogged – not water alerts.
    These warn you ahead of the vehicle alerts that there is a problem coming up.

    Do you know of an Australian supplier?