OBD codes / P0042
P0042
ModerateO2 Sensor Heater Control Circuit (Bank 1, Sensor 3)
P0042 is safe to drive with indefinitely from a mechanical standpoint — this sensor only audits the second catalytic converter and has no hand in fueling. Handle it as housekeeping: repair before your next emissions test, and keep in mind the lit warning light could mask a new, unrelated fault.
What this code means
Sensor 3 is the least familiar oxygen-sensor position: it exists only on vehicles with a second catalytic converter in line on that side of the exhaust, where it sits after that second converter to monitor its performance. P0042 flags a fault in the heater control circuit of that third sensor on Bank 1 — the exhaust path serving the side of the engine with cylinder 1. As with every O2 sensor, the heater's job is to bring the ceramic sensing element to operating temperature within seconds of a cold start so monitoring (and, for upstream sensors, closed-loop fuel control) can begin promptly. A generic circuit code like this typically means the heater circuit is open or unresponsive — most often a failed heater element inside the sensor, and otherwise a blown fuse or damage to the long underbody wiring run that reaches this far-back sensor. If your car doesn't have a third sensor on that side, this code shouldn't appear on it at all. Driving feel is unchanged; the cost of ignoring it is a standing check-engine light and an incomplete converter monitor.
Symptoms you might notice
- Check-engine light, usually with zero drivability symptoms
- No real-world change in power or fuel economy
- Failed emissions inspection while the code is active
What it costs to fix
Typical range: $150–$425 · about ~0.5–1.5 hrs of labor
| If the cause is… | Part | Labor |
|---|---|---|
| Third (downstream) oxygen sensor | $30–$200 | ~0.5–1.5 hrs |
| Wiring / connector repair | $10–$120 | ~0.5–1.5 hrs |
| Heater-circuit fuse | $1–$10 | ~0.1–0.3 hrs |
Sensor 3 lives well back along the exhaust, so most of the cost variation is labor and access — a lift makes it quick, driveway conditions make it slower. The sensor itself is usually a simpler downstream type on the cheaper end of the range.
The price swings on which cause it turns out to be — so confirm the cause before paying. Diagnose P0042 for my exact vehicle →
Frequently asked
My car only shows two O2 sensors per side — why P0042?
P0042 belongs to a third sensor that only exists on vehicles with two catalytic converters in series on that bank. If you're seeing this code, your exhaust almost certainly has that layout — check behind the second converter. It's not a code that applies to a two-sensor exhaust.
Is P0042 urgent?
No. It's a heater fault in a purely monitoring sensor at the tail end of the emissions system. Drive normally and schedule the fix; the only firm deadline is an emissions inspection, which an active code will fail.
What causes P0042 most often?
The heater element inside the sensor burning out from age — heater failure is the most common way any O2 sensor dies. Because this sensor's wiring makes a long run under the car, harness damage and corroded connectors are a stronger second-place cause here than on engine-bay sensors.
Will replacing the sensor definitely clear P0042?
Only if the sensor was the problem. Before buying one, it's smart to check the heater fuse and unplug the connector to look for corrosion — the same code appears whether the failure is in the sensor, its wiring, or its power feed.
Seeing P0042 on your car? Get a diagnosis specific to your exact year, make and model.
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Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.