P0142
O2 Sensor Circuit Malfunction (Bank 1 Sensor 3)P0142 is a generic OBD-II powertrain diagnostic trouble code: O2 Sensor Circuit Malfunction (Bank 1 Sensor 3). It is logged by the engine control unit when the o2/lambda monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0142 means
P0142 is stored when the Engine Control Module (ECM) detects that the output voltage from the oxygen sensor at Bank 1, Sensor 3 (B1S3) is outside the expected range during a test cycle. Sensor 3 on Bank 1 is positioned downstream of the second (final) catalytic converter in exhaust systems that use multiple converter stages — a configuration found on many V6, V8, and inline-6 engines with dual-cat or underfloor secondary converters.
The oxygen sensor produces a voltage signal between approximately 0.1 V (lean exhaust) and 0.9 V (rich exhaust). When the ECM sees a signal that is stuck low, stuck high, or fails to respond within the expected voltage window over a monitored period, it sets P0142. Because Sensor 3 sits after the final catalyst, its signal is used for additional catalyst monitoring and for gross fuel-trim verification rather than direct closed-loop fuelling — meaning most drivers will not notice a drivability change when this code is active.
Common causes include a failed O2 sensor element, a shorted or open heater circuit (which prevents the sensor reaching the ~300 °C operating temperature needed for accurate readings), wiring damage from exhaust heat, and exhaust leaks upstream of the sensor that introduce excess oxygen into the stream, producing a falsely lean (low voltage) signal. Contamination from coolant or oil leaks can also poison the sensor's ceramic element and cause an erratic or dead output.
While the code does not typically produce noticeable symptoms beyond the MIL, it can mask a degrading secondary catalytic converter and may result in an emissions inspection failure. Repairs usually involve replacing the downstream sensor or repairing its wiring harness.
Common causes
Most-frequently reported root causes when P0142 is logged.
-
1
Failed O2 sensor element (internal open or short)
-
2
Damaged wiring or connector at the B1S3 sensor (heat or abrasion)
-
3
Exhaust leak upstream of Sensor 3 introducing excess oxygen
-
4
Failed or open heater circuit preventing sensor warm-up
-
5
Sensor contaminated by oil or coolant entering the exhaust
-
6
Corroded sensor connector pins causing high resistance
-
7
Short to ground on the signal wire
-
8
Faulty ECM/PCM (rare — rule out external causes first)
Symptoms drivers notice
How to diagnose P0142
A typical diagnostic flow when this code is present.
-
1
Scan for all stored and pending codes; note if P0142 appears alongside catalyst efficiency codes that may indicate a deeper exhaust issue.
-
2
Locate B1S3 on the vehicle — it is typically the sensor bolted to the exhaust pipe or pipe joint downstream of the last catalytic converter on the Bank 1 side.
-
3
Inspect the sensor connector and harness for heat damage, chafing against exhaust components, or corrosion at the connector pins.
-
4
With the engine at operating temperature, use a scan tool to monitor the live voltage of B1S3; it should fluctuate between 0.1 V and 0.9 V or hold a steady post-cat voltage (~0.6–0.7 V). A reading stuck at 0 V or above 0.9 V indicates a sensor or wiring fault.
-
5
Measure heater circuit resistance at the sensor connector (2–30 Ω typical); an open reading (OL) confirms a failed heater element.
-
6
Check for exhaust leaks at manifold gaskets, flex joints, and welds upstream of the sensor; repair any found before replacing the sensor.
-
7
Replace the sensor if all external circuits and wiring test good, then clear codes and perform a complete drive cycle to confirm resolution.
Related powertrain codes
- P0040 — Upstream Oxygen Sensors Swapped From Bank To Bank
- P0041 — Downstream Oxygen Sensors Swapped From Bank To Bank
- P0130 — O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)
- P0131 — O2 Sensor Circuit Low Voltage (Bank 1 Sensor I)
- P0132 — O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)
- P0133 — O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
Frequently asked questions
Does P0142 affect how my engine runs day-to-day?
Usually not noticeably. The ECM does not use B1S3 for primary fuel trim corrections, so smooth idling and normal acceleration are typically maintained. The main consequences are an illuminated MIL and compromised secondary catalyst monitoring.
Can an exhaust leak set P0142?
Yes. A leak upstream of Sensor 3 draws in ambient air, artificially leans out the exhaust gas sample, and causes the sensor to output a sustained low voltage that the ECM interprets as a circuit malfunction. Sealing the leak often resolves the code without replacing the sensor.
How does P0142 differ from P0141 or P0144?
P0141 is a heater-circuit fault for B1S2; P0142 is a general signal-circuit malfunction for B1S3; P0144 is high-voltage signal on B1S3. They all relate to Bank 1 downstream sensors at different positions or failure modes.
Is P0142 more common on certain engine configurations?
It is most prevalent on vehicles with dual catalytic converter stages (e.g., many V6/V8 engines and some turbo inline-engines), as these are the only configurations that use a third O2 sensor position. Four-cylinder engines with a single cat rarely have a Sensor 3.
Disabling P0142 in software
RaceTune can permanently disable P0142 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
Got P0142 in your scan?
Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.
Upload your file