P0426–P043B Catalyst Temperature Sensor Circuit Range/Performance Bank 1 Sensor 1

Which code is which

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

The definition

P0426: Catalyst Temperature Sensor Circuit Range/Performance Bank 1 Sensor 1.

P0426 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. Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
2. Check the connector at the part for corrosion, and that it is fully seated.
3. Clear the code and see whether it returns, and under what conditions.

What actually causes this on a VAG

This code refers to a catalyst temperature sensor - an exhaust gas temperature probe screwed into the exhaust that tells the engine control unit how hot the gases actually are. The official definition is bank 1, sensor 1, meaning the first such sensor counted from the engine on that side; the second bank of a V6, V8 or W engine has its own separate codes. On VAG vehicles these probes are common on diesels, around the turbo, the oxidation catalyst and the particulate filter, and on turbocharged petrol engines, where they protect the turbo and catalyst from overheating. Many older naturally aspirated VAG petrols have no such sensor at all and work out catalyst temperature in software instead. 'Range/performance' means the signal is electrically present and looks plausible, but the value it reports does not agree with what the control unit expects - a lazy, drifting or stuck reading rather than a dead circuit.

Most likely causes, in order

1. Damaged wiring or a corroded connector at the sensor. Free to check and genuinely common, so do it first. The tail from the sensor runs through the hottest, dirtiest part of the car and is usually clipped near a heat shield. Chafed insulation, a melted section or green pins in the plug all produce erratic readings. Wiggle the loom with the live reading on screen and watch for the value jumping - if it moves when you move the cable, you have found it without spending anything.
2. The sensor has aged and drifted. These probes live in extreme heat and cycle from cold to glowing repeatedly, so they do wear out, and this is where a lot of these faults end up. The useful test costs nothing: with the engine stone cold after standing overnight, look at the live readings from all the exhaust temperature sensors at once. They should all sit close to the outside air temperature. One that disagrees with the others when everything is cold is faulty. Be aware of what that test does not prove - a sensor can read correctly when cold and still drift once hot, so all of them agreeing does not clear them, it just means you need to watch the readings warm up together.
3. Sensor loose, wrongly positioned or the wrong part. If the tip is not properly in the gas stream - not fully screwed in, sitting in the wrong boss, or a non-specified aftermarket sensor with a different response curve - it will read low and slow even though nothing is electrically wrong. Very common after exhaust, turbo or particulate filter work.
4. The exhaust really is at an unexpected temperature. Less likely, but the control unit is comparing the sensor against its own model of what the temperature should be. A separate fault affecting combustion or regeneration can make an honest sensor look wrong, which is why it is worth reading the rest of the fault memory before condemning the probe.

Worth knowing: Do the cold-start comparison before buying anything - it costs nothing and it settles the question in one direction at least. And be aware these sensors seize solid in the exhaust: expect penetrating fluid, heat and patience, because snapping the tip off turns a sensor job into removing the section of exhaust it is stuck in. Let the car cool properly first; exhaust components hold enough heat to burn long after the engine is switched off.

Common questions

What does P0426 mean?

P0426 means Catalyst Temperature Sensor Circuit Range/Performance Bank 1 Sensor 1. The sensor still works, but its reading has drifted outside the window the ECU expects. That usually means a tired sensor or a problem with what it is measuring, not a broken wire.

Can I drive with P0426?

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

• EA888 1.8 and 2.0 TSI Problems: An Owner’s Guide
• Rough Idle on a VW or Audi: Causes and How to Narrow It
• White, Blue or Black Exhaust Smoke: What Each Colour Means
• VW or Audi Overheating: What to Do Now and What Causes It
• Why the same fault code has completely different causes