U3004 Moderate Generic / SAE

U3004: Accessory Power Relay

Network and Communication Control Module Power and Ignition Supply

What to do

The symptom is usually accessories — radio, screens, sockets — dying or not coming back after starting.

A cheap and genuinely useful test: the fuse box often contains several identical relays. Swap this one with another of the same type and see whether the fault moves with it. If it does, you have found it for the price of nothing.

If it works when cold and fails when the car has warmed up, say so — that pattern points straight at the relay rather than the wiring, and it saves diagnostic time.

The car should drive normally, since this circuit is about accessories rather than the engine. If it does not, there is something else stored and this is not the whole story.

What this code means

A module is reporting a problem with the accessory power relay.

This range is different from every other network family, and the difference decides how it should be diagnosed.

U01xx and U02xx say a module went silent. U04xx and U05xx say a module is talking nonsense about something it watches. This range says a module is reporting a problem with its own foundations — its supply, its earth, or the ignition state that tells it whether the car is even meant to be awake.

Which makes these codes upstream of almost everything else. A module with a marginal earth does not fail cleanly; it reports implausible values about every sensor it reads, sets codes in unrelated systems, and drops off the bus intermittently. A scan tool showing thirty codes across five systems very often has one of these underneath it.

So read this family first and fix it first. Clearing a page of downstream codes before the supply fault is repaired produces a page of downstream codes again by the end of the week.

And start with the battery, genuinely. Not as a formality — a twelve-volt battery weak enough to sag during cranking, or a terminal with corrosion under a clamp that looks tight, reproduces this entire family exactly. It is the cheapest thing to eliminate and by some distance the most common cause.

The accessory relay is what feeds everything that runs with the key on but the engine off — radio, screens, sockets, and on many cars the climate blower.

Its distinctive job is dropping out during cranking. The starter draws hundreds of amps and pulls system voltage down, so the car deliberately sheds accessory loads while the engine turns over. That is why the radio cuts out as you start the car — and why a fault here often presents as accessories that never come back afterwards.

Relays fail in a way worth knowing: mechanically, and gradually. The contacts inside carry real current and erode, so the failure is usually intermittent before it is permanent, and heat makes it worse. A relay that works cold and not when the car is warm is describing itself.

Which makes the classic test genuinely useful: swap the relay with an identical one elsewhere in the box — a horn or fog lamp relay is often the same part — and see whether the fault follows it. That costs nothing and settles the question.

Likely causes

Start here

Low system voltage · easy
Corroded earth connection · moderate
Blown fuse or failed supply · easy
Failed power or earth supply to the module · moderate
Corroded or loose connector · easy
Faulty module power relay · moderate

Less common

Damaged wiring harness · moderate

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

Be the first to write this up

Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.

What makes one worth reading:

The car — make, model, year and engine. The same fault behaves differently on each.
What you measured, not only what you replaced.
What actually fixed it — or that nothing did, which is worth knowing too.