P0042 code: O2 Sensor Heater Control Circuit (Bank 1, Sensor 3)
P0042 means o2 Sensor Heater Control Circuit (Bank 1, Sensor 3). The code says where to look, not what failed — the fix depends on which cause your car actually has.
What P0042 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.
P0042 symptoms
- 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 P0042
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 →
How to fix P0042
Moderate — doable at home with common tools and patience
- Confirm the car really has a sensor 3
Sensor 3 exists only when Bank 1 carries a second converter in line, and it sits after that second unit — usually the furthest-back sensor on the cylinder-1 side. Count the sensors as you follow that pipe rearward, and check service information for the layout. Testing the wrong one is easy when three sensors share a single exhaust path.
- Check the shared feed and the code list
Inspect the heater fuse, and read every code: P0042 alongside P0146 suggests the sensor is cold and silent because its heater is dead, while several heater codes at once point at a shared fuse or feed. A lone P0042 keeps the focus on this rearmost sensor and its lead.
- Walk the long harness toward the rear
The lead to a third sensor is among the longest on the car, clipped along the floor pan toward the back. Look for clips rusted away, wiring scraped by debris thrown up from the road, and a plug caked in salt. A harness hanging low enough to snag on speed bumps is a strong suspect before the meter even comes out.
- Test the element, then the feed at the plug
Unplug the sensor and measure across its heater contacts with everything cold — open means the element is gone. Then back-probe the harness half with the engine running for battery voltage. A sound element with no voltage sends you back up the lead; a dead element with good voltage means a new sensor.
- Free the sensor without stripping the bung
Sensors this far back rust in place more than they heat-seize. Soak the threads over a day or two, warm the pipe with a short run, and use a slotted O2 socket with steady pressure rather than jerks. Run a thread chaser through the bung before fitting the new sensor so it seats fully and seals.
DIY or shop? Diagnosing P0042 is approachable — a fuse, a visual check and two meter readings — and the far-back position at least means no engine-bay clutter to work around. What tends to push it to a shop is corrosion: a sensor after the second converter on an older car can be fused to the pipe, and stripping the bung means welding. If the harness is damaged mid-run, splicing it properly under the car is slow work on your back too.
Tools for the DIY steps: Multimeter · Jack stands or a lift · Slotted O2 sensor socket · O2 sensor thread chaser · Penetrating oil
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for P0042.
Start a case with P0042 and your car. Redline weighs these causes against what your car shows, and names the check that would settle it before you buy parts.
Can I drive with P0042?
Redline can’t determine from a symptom or code alone whether your car is safe to drive.
P0042: common questions
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?
Not as repairs go. It's a heater fault in a purely monitoring sensor at the tail end of the emissions system. 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.
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like P0042 from your phone. See our guide to the best OBD2 Bluetooth adapters. Run a repair shop? See Redline for shops.
Generic OBD-II reference. Manufacturer-specific behavior varies. Confirm with a scan tool and, for safety-related codes, a professional inspection. Cost ranges are reference ranges, not quotes.
Seeing P0042 on your car?
Every problem becomes a Case. Tell Redline what the car is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.