P0189: Fuel Temperature Sensor "B" Circuit Intermittent

P0189 records a fault that has already repaired itself. The circuit dropped out, the controller noticed, and by the time anyone connects a scan tool the reading looks entirely reasonable. Intermittent faults are difficult everywhere, but this one carries a complication the rest of the family does not: on a great many vehicles the second fuel temperature sensor is inside the fuel tank, on the pump and sender module, sealed behind a locking ring and a gasket under the rear seat or the boot floor. You cannot flex that harness. You cannot heat it, mist it with water, or tug on its terminals. The standard provocation techniques for an intermittent fault assume you can put your hands on the circuit, and here you often cannot.

So the method has to change from provocation to correlation, and the variable to correlate against is fuel level. That is not an arbitrary choice — it is the one thing about a tank-mounted circuit that changes continuously while the vehicle is used. A connector or a terminal that is submerged at three-quarters of a tank is in vapour at a quarter, and the electrical behaviour of a marginal joint is not the same in both states. A module that is supported by the fuel around it moves differently when the tank is nearly empty. And fuel sloshes: hard cornering, heavy braking and a steep driveway all move several inches of liquid across the top of the sender assembly in a second. A fault that shows up on roundabouts, or only in the last quarter of a tank, or reliably on the drive home when the gauge is low, is telling you where it lives.

The practical version of this is to log rather than to poke. Leave a scan tool recording fuel temperature and fuel level together over several days of ordinary driving, then look at where the dropouts fall on the level axis. If they cluster, the tank module is implicated and the case for opening it is made on evidence rather than on hope, which matters when the job involves removing seats or trim and dealing with a live fuel system. If the dropouts are scattered evenly across all fuel levels, the fault is probably not in the tank at all and the long underbody run deserves the attention instead.

There is a second thing worth knowing before opening a tank module, and it concerns what corrosion looks like in there. Terminals under the bonnet corrode in a way most people recognise: green or white oxide, powdery, obviously wrong. Inside a fuel module the chemistry is different. Fuel is a solvent, and pump-grade gasoline blended with ethanol is a considerably more aggressive one than straight hydrocarbon — it holds water in suspension, and that water ends up in contact with terminal plating that was only ever specified for a dry environment. The result is not powder but etching: dull, pitted, grey-brown contact faces that look merely dirty until you compare them against a new part. Wiping them and refitting restores contact for a while and the fault returns, which is why intermittents from fuel modules have a reputation for being fixed twice. Replacing the affected terminals or the module pigtail is the repair that lasts.

One final point of interpretation. Because this code can sit dormant for weeks, its presence alongside a stored "A" channel code from months earlier does not mean the two faults are related. Check the mileage stamps or the count of warm-up cycles since each code set before assuming a common cause — treating an old, unrelated code as evidence has sent plenty of diagnoses in the wrong direction.