U0673 Moderate Generic / SAE

U0673: Lost Communication With Ion Sense Module

Network and Communication Lost Communication with Sensors and Actuators

What to do

The car drives normally but may feel slightly less sharp, and may use a little more fuel — that is deliberate.

Your engine uses the spark plugs themselves as sensors: just after each spark, it measures electrical current through the burning mixture to tell exactly how the fuel is burning. Losing that measurement means the engine falls back to safer, cruder methods, so it retards the timing and gives itself a margin.

Have the spark plugs and coils inspected as part of the diagnosis. The measurement runs through them, so worn plugs, a cracked coil boot or moisture in a plug well produce faults that look exactly like a failed module — and they are far cheaper.

If the scan tool also lists several codes in the P289x range, mention it. Those are the per-cylinder version of the same system, and they point at the same place.

No urgency, but do not leave it indefinitely — an engine running on fallback timing is being protected rather than optimised, and the fuel cost is real.

What this code means

The ion sense module has stopped communicating.

This range exists because parts stopped being parts.

A lost-communication code at U01xx or U02xx means a control module went silent — a computer, with a processor and a name. This range says the same thing about a coolant pump, a valve, or a sensor, and that sentence only became possible when those components acquired controllers of their own.

Most of them are not on the main CAN bus. They hang off a LIN sub-bus — a single-wire, low-speed network that a master module runs to a chain of small devices, precisely because giving every actuator a full CAN interface would be absurd. LIN is cheap, slow, and has one property that dominates diagnosis: it is a chain.

Which produces the most useful fact about this whole family. The fault is very often not in the part named, and not even in its wiring. A LIN chain shares one wire, one supply and one master:

One device silent, the rest talking — the fault is that device or its connector.
Several devices on one chain silent together — the fault is the shared wire, the shared supply, or the master module that runs them. Replacing any one of the named parts fixes nothing.

So read every other stored code before touching anything. On this family more than most, the pattern across codes is the diagnosis, and a single code in isolation means something quite different from three that arrived together.

These devices are also, physically, the ones in the worst places — bolted to engines, in wheel arches, under the front bumper. Heat, vibration, salt and water reach them in a way they never reach a module behind a dashboard, so connector corrosion is a leading cause here rather than an afterthought.

Ion sensing is one of the cleverest ideas in engine management, and almost nobody has heard of it: the spark plug is used as a sensor after it has finished being a spark plug.

Immediately after the spark, the burning mixture in the cylinder is electrically conductive — combustion produces ions. Apply a small voltage across the same plug electrodes and the current that flows describes what is happening inside the cylinder in real time.

Which gives an engine several measurements it would otherwise need extra hardware for, and gives this module its job:

Knock detection, per cylinder, from inside the combustion chamber rather than by listening to the block through a microphone.
Misfire detection, directly — a cylinder that did not burn produces no ion current at all, which is far more certain than inferring it from crankshaft speed.
Combustion timing, letting the controller place peak pressure optimally instead of relying on a fixed map.

So losing it costs the engine its most precise feedback, and the response is the conservative one: retarded ignition timing, reduced boost where fitted, and a fallback to the cruder detection methods. The car runs, slightly less efficiently and slightly less sharply, and protects itself with a margin rather than with measurement.

The corpus carries the rest of this technology at P2890–P28C2, the per-cylinder ion current sense circuit codes. Those codes have only one source today and remain undocumented — a gap worth knowing about if a scan tool shows several of them alongside this one.

Because the module works through the ignition system, ignition faults imitate it. Worn plugs, a cracked coil boot or moisture in a plug well all corrupt the tiny currents being measured, so a plug and coil inspection is worth more here than it sounds.

Likely causes

Start here

Less common

Failed power or earth supply to the module · moderate
Heat damage to sensor or wiring · moderate
Damaged spark plug wire or boot · moderate
Water in the spark plug well · 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.