U0070 Moderate Generic / SAE

U0070: Vehicle Communication Bus E (-) Low

Network and Communication Bus Signal and Bus Off Emissions related

What to do

Expect a group of systems to have stopped or to be misbehaving.

As at U0067, notice what the car is doing when it happens — rough roads, pulling away, a door slammed. Each points at a different part of the car and it is the one thing you can supply that a garage cannot.

But there is one cause worth raising first here, because it is cheap to check and easy to miss: the battery.

The computers need a steady supply to put the right voltages on the network. As the battery weakens, those levels fall with it — so a network that is completely healthy can fail its own checks while cranking a tired battery, or when a heavy load drags the system voltage down.

That is not a wiring fault at all.

So have the battery and charging system tested early rather than late. A garage chasing a network fault on a car with a failing battery can spend a great deal of money finding nothing — and "it started around the time the car got harder to start" is exactly the observation that prevents it.

What this code means

The negative line of vehicle communication bus E is being held low. Everything at U0067 applies — including that the observation which identifies an intermittent short is what the vehicle was doing rather than what failed.

One addition belongs here, and it is the cause U0064 lists that people find hardest to accept: low system voltage.

A transceiver needs its supply to hold the bus at the right levels. As system voltage falls, the levels it can produce fall with it — and a network that is entirely healthy at 12.6 volts can fail its own checks while cranking a tired battery, or while a heavy load pulls the system down.

Which produces a fault that is genuinely not a wiring fault at all, and the signature is in the stored conditions: codes logged consistently at low voltage, often during cranking, on a vehicle whose network is otherwise clean.

So the battery and the charging system belong in the elimination, and early rather than late — because they are cheap to test, and because a workshop chasing a phantom bus fault on a vehicle with a failing battery can spend a great deal of somebody's money.

Likely causes

Start here

Less common

Corroded earth connection · moderate
Poor wiring splice in an aftermarket installation · 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.