OBD-II Code
P0042
Severity
Not yet reviewed
Nobody on our side has assessed this code yet, so we are not going to guess at how serious it is.
Can you drive it
We cannot say
Treat it as unknown until a shop reads it with live data. If the check-engine light is FLASHING, stop driving as soon as it is safe to — that means an active misfire, whatever the code.
Indicative cost (USD)
$80–$480
From this code's diagnostic notes. Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
P0042 means the heater circuit on Bank 1 Sensor 3 has a generic fault — and the immediate question is which O2 is even called Sensor 3, because that number only exists on vehicles with three O2 sensors per bank (one pre-cat, one between cat sections, one post-cat) or on vehicles with a wide-band primary plus a switching mid-cat sensor plus a downstream switching sensor.
How a shop works this
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.
Note every code that is set, not just this one.
Codes that appear together usually narrow the cause faster than any one of them read alone.
Read the freeze frame — the conditions the fault was recorded under.
Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.
Confirm the fault is current rather than history.
Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.
Then follow the diagnostic notes further down this page, in the order they are written.
They are written in the order a shop works this code, cheapest checks first. Where they name a component, inspect its wiring and connector before replacing it.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Check engine light
Read the full diagnostic procedureP0042 means the heater circuit on Bank 1 Sensor 3 has a generic fault — and the immediate question is which O2 is even called Sensor 3, because that…
P0042 means the heater circuit on Bank 1 Sensor 3 has a generic fault — and the immediate question is which O2 is even called Sensor 3, because that number only exists on vehicles with three O2 sensors per bank (one pre-cat, one between cat sections, one post-cat) or on vehicles with a wide-band primary plus a switching mid-cat sensor plus a downstream switching sensor.
Cheapest-first ladder
- confirm the vehicle actually has a Bank 1 Sensor 3 before chasing wiring — many scan tools mis-label sensors on multi-cat exhausts, and what one tool calls Sensor 3 another calls Sensor 2 on the same vehicle; consult the FSM exhaust diagram before pulling parts.
- Backprobe the heater supply pin at the Sensor 3 connector — must read B+ (12.5-14.0V) key-on; measure heater resistance with the connector unplugged (2-15 ohms cold, with most sensors in the 5-9 ohm range); clamp an inductive ammeter on the heater supply lead with engine running warm and confirm 0.5-2.0A current draw at 12V. If supply voltage is missing, trace back to the heater relay/fuse; if heater resistance is open or shorted, replace the sensor; if both check out but the code persists, the issue is the ECM low-side driver itself (rare but real on aged ECMs).
- The expensive misdiagnosis on P0042: techs replace what they THINK is Sensor 3 based on scan-tool labeling and find the new sensor doesn't clear the code because the actual Sensor 3 on this platform is the mid-cat sensor, not the post-cat — verify sensor identity against the FSM exhaust schematic, not the scan tool's label.
Vehicle-specific patterns2005-2011 Mercedes-Benz M272 3.0/3.5L V6 (C-Class, E-Class, ML350, R350) and M273 V8 (E550, ML550, S550) on certain export-market configurations carr…
- 2005-2011Mercedes-Benz M272 3.0/3.5L V6 (C-Class, E-Class, ML350, R350) and M273 V8 (E550, ML550, S550) on certain export-market configurations carry a Bank 1 Sensor 3 on the secondary cat — the harness routes under the transmission tunnel and chafes against the driveshaft tunnel seam, opening the heater circuit at 80k+ miles.
- 2003-2010BMW N62 4.4/4.8L V8 (5/6/7 Series, X5) ships with four O2 sensors total (two per bank) and the rear sensor on Bank 1 is what the BMW scan tool labels Sensor 3 in some firmware versions — the heater connector at the rear of the engine sees exhaust heat soak and the pins anneal/loosen.
- 2010-2013Range Rover/Range Rover Sport with the Jaguar AJ-V8 5.0L (also seen on F-Type and XK) sees P0042 from a known underbody harness chafe just ahead of the differential.
- 2008-2014Audi/VW 4.2L FSI V8 (S5, RS5, Q7) sees P0042 from corrosion at the rear sensor connector after winter salt exposure.
Estimated repair$80 to $480.
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