P242B: Exhaust Gas Temperature Sensor Circuit Range/Performance Bank 1 Sensor 3
What to do
Mention whether the car has been attempting particulate filter regenerations unusually often — a cooling fan running on after you park, a hot smell, or the trip computer showing worse economy than normal. That pattern is the useful clue here and points at this sensor rather than at the filter.
Check the engine oil level yourself before the appointment. Repeated failed regenerations put diesel into the sump, and an oil level that has risen above full is a genuinely urgent finding rather than a curiosity — it can run the engine on its own oil.
Mention any recent exhaust work. A sensor not seated properly after a filter or pipe change produces exactly this code, and that is a much cheaper fix than a new sensor.
What this code means
The third exhaust gas temperature sensor on bank 1 is reporting a temperature that does not fit what the rest of the exhaust is doing.
A modern diesel exhaust carries several temperature sensors in series, and their whole purpose is to be compared with one another. Sensor 3 usually sits after the particulate filter, where it confirms that a regeneration actually reached the temperature it was supposed to reach.
Range/performance means the signal is electrically plausible but wrong in context. The circuit is not open and not shorted — the number is simply not credible next to the sensors either side of it, or it fails to move when the exhaust demonstrably heats up.
The commonest genuine cause is a sensor that has drifted with age, which these do: they live in soot and heat cycle to several hundred degrees thousands of times.
But check the physical installation before condemning it. A sensor that has worked loose in its boss, or one whose tip sits in a different part of the gas stream after an exhaust repair, reads low and steady for entirely mechanical reasons.
The consequence to explain is regeneration. The module uses this reading to decide whether a regeneration succeeded. A sensor reading low makes every regeneration look like a failure, so the vehicle keeps attempting them — which burns fuel, dilutes the engine oil, and eventually blocks the filter it was trying to protect.
Likely causes
Start here
- Sensor drifted out of calibration · moderate
Less common
- Sensor not refitted correctly after a filter change · easy
- Exhaust leak upstream of the sensor · moderate
- Heat damage to sensor or wiring · moderate
- High resistance in sensor wiring · moderate
Symptoms
Components involved
Information, not instruction
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Community experience
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