P0642–P0698 Sensor Reference Voltage A Circuit Low

Which code is which

Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.

The definition

P0642: Sensor Reference Voltage A Circuit Low.

P0642 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.

What to check first

Cheapest and fastest checks first:

1. Unplug the sensor. If the reading jumps to full scale, the wiring is fine and the sensor has failed low.
2. Check the sensor's 5V supply and its earth before condemning the sensor itself.

What actually causes this on a VAG

The engine ECU generates its own regulated 5-volt supplies and shares them out among groups of sensors - things like throttle and pedal position, boost and manifold pressure, fuel pressure and EGR position. Most ECUs have more than one of these reference circuits; this code is about circuit A, and says that supply has been dragged down below where it should be. Because a whole group of sensors depends on it, the usual experience is several unrelated warning lights and codes appearing together, often with reduced power. The important thing to understand is that this is one fault affecting many sensors, not many faults.

Most likely causes, in order

1. One sensor, or its wiring, shorting the supply to earth. The commonest cause by far. Something on the shared line has gone short internally, or the loom has rubbed through onto the block, a bracket or a heat shield. The practical test is to unplug the sensors on that circuit one at a time with the ignition on and watch the reference voltage: when it recovers, the last thing you unplugged is on the faulty branch. Two caveats - unplugging sensors stores open-circuit codes of their own, so clear them and re-check afterwards; and a branch that frees the supply does not automatically mean the sensor is bad, so inspect that length of loom before condemning the part.
2. Water or corrosion inside a connector. Moisture bridging pins inside a plug does exactly the same job as a short. Look at connectors in the exposed places: under the scuttle or plenum, at the back of the engine, low down at the front, and anywhere a blocked drain or a leaking screen surround has let water sit. Dry, clean and reseal rather than replacing parts.
3. Chafed or pinched harness. A loom trapped against a hard edge or crushed under a bracket during previous work will eventually break through the insulation. Check anything that has recently been off the car, and inspect where the harness passes over the engine and along the bulkhead.
4. A failed voltage regulator inside the ECU. Genuine but rare, and the right conclusion only once every sensor on the circuit has been unplugged and the wiring proved clear. Nobody should be selling you an ECU on the strength of this code alone.

Worth knowing: Do not replace the sensors that are throwing codes. When P0642 is stored alongside a cluster of unrelated sensor faults, those sensors are almost always innocent - they are simply being starved of the supply they need, and they will all report normally again the moment the real short is found. Chase the shared wire, not the list of codes. While it is stored, expect the car to hold itself in reduced power, so allow for that in traffic until it is fixed.

Common questions

What does P0642 mean?

P0642 means Sensor Reference Voltage A Circuit Low. The voltage on this circuit is sitting near zero. Typically a short to earth, or a sensor that has failed to its low end.

Can I drive with P0642?

The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.

Related reading

• Battery Keeps Going Flat on a VW or Audi: Finding the Cause
• VW or Audi will not start: working out which problem you have
• Several warning lights on at once: usually one cause, not many
• Why the same fault code has completely different causes