What's the difference between P0136 and P0140?

P0136 means the downstream O2 sensor on bank 1 is reading a low voltage — there's a signal, it's just persistently below the expected range. P0140 means there's no useful signal at all — the voltage is flat, stuck at one value, or completely unresponsive. P0136 often points to the sensor or wiring shorted to ground. P0140 more often points to a dead sensor, an open circuit, or a failed heater preventing the sensor from warming up.

Is this a dead sensor or a wiring problem?

Hard to tell without checking. Easiest way: pull the connector at the sensor and check resistance across the heater terminals (should be 4-15 ohms). If that's open or way out of spec, the sensor is dead. If the heater resistance is good, backprobe the connector with the engine off and look for battery voltage at the heater supply pin — missing voltage means a wiring or fuse problem, not a sensor problem. About 50% of P0140 codes turn out to be the sensor, 30% wiring/connector/fuse, and the rest a mix of catalyst and PCM issues.

Can I keep driving with P0140?

Yes, in most cases. Downstream O2 sensors don't drive fuel trim, so you won't feel a change in power or smoothness. You'll lose a small amount of fuel economy and you won't be able to pass an emissions inspection until the code is cleared. If the underlying cause is a failed heater drawing current funny, fix it sooner — heater failures occasionally take out the fuse and other items sharing the circuit.

How much will P0140 cost to repair?

Cheapest case: under $5 for a blown fuse if that's the issue. Typical case: $150-400 for downstream O2 sensor replacement. Wiring repair: $50-500. Heater fuse plus a new sensor: around $250 if the heater took out the fuse on its way out. Diagnose before buying parts — many P0140 codes resolve below the worst case.

OBD-II trouble code

P0140: O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 2)

The downstream oxygen sensor on bank 1 isn't producing a signal at all — the voltage is flat, stuck at a single value, or the PCM sees no response when it would expect one. Different failure mode from P0136 (low voltage): no signal, not a low one.

Quick facts

System: Powertrain
Category: Oxygen Sensor
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $5 – $500
DIY difficulty: Intermediate DIY

What does P0140 mean?

P0140 is easy to confuse with P0136, and the distinction matters for diagnosis. P0136 says the downstream sensor on bank 1 is reading low — there's a signal, it's just persistently below where it should be. P0140 says there's no useful signal at all: the voltage is flat, completely unresponsive to changing conditions, or stuck at a single value that doesn't move when the engine state changes. The PCM expects to see at least some movement from a working downstream sensor; complete inactivity is what P0140 reports.

Three categories of root cause produce P0140. The first and most common is a dead sensor — the sensing element has aged out, internal contamination has killed the response, or the heater inside the sensor has failed badly enough that the sensor never reaches operating temperature. Second is the wiring path — an open circuit somewhere between the sensor and the PCM, a broken pin in the connector, or chafe damage on a signal wire. Third, and worth ruling out before condemning the sensor, is the heater circuit itself. If the sensor's heater never powers up (because of a blown fuse, broken wire on the heater supply side, or a PCM-side driver failure), the sensor never warms up enough to produce a signal, and the PCM reports no activity. That last case often sets a heater-circuit code (P0141) alongside P0140 — if you see both, fix the heater first.

Unlike upstream O2 codes, P0140 doesn't directly affect fuel trim or driveability. The downstream sensor's job is monitoring catalyst efficiency, not running closed-loop fueling. So the symptoms are subtle: a Check Engine Light, a small fuel economy hit, failed emissions readiness, and not much else. That subtle symptom profile is why P0140 often goes unnoticed for a long time — owners only discover it when a smog inspection comes up.

Common causes

Failed downstream O2 sensor on bank 1 — dead sensing element or completely failed heater
Open circuit in the signal wire between the sensor and the PCM
Broken or backed-out pin at the sensor connector
Blown fuse on the O2 sensor heater supply circuit
Failed heater inside the sensor (often sets P0141 alongside P0140)
Chafed wiring or damaged harness near the catalytic converter heat shield
Corroded sensor connector preventing reliable signal return
PCM driver circuit failure (rare but possible on high-mileage modules)

Symptoms

Check Engine Light on
Slight loss of fuel economy
Failed emissions test or emissions readiness not complete
Catalyst monitor not ready on a smog check
No noticeable change in power, idle quality, or driveability
Sometimes a faint exhaust smell on cold start (if a heater failure is preventing the sensor from warming up)

Diagnostic steps

1.Pull all current and pending codes. P0140 alongside P0141 means the heater circuit is the underlying issue — fix that first and P0140 often resolves with it.
2.Read live data for bank-1 sensor-2 voltage. A reading that never moves from a single value (e.g., stuck at 0.0V or 0.45V) confirms 'no activity'.
3.Inspect the sensor connector for oil contamination, corrosion, backed-out pins, or melted insulation.
4.Check the relevant O2 sensor heater fuse (often labeled in the underhood fuse box). A blown fuse is a 30-second fix and a common cause.
5.With the connector unplugged, check resistance across the sensor's heater terminals. Most read 4-15 ohms; an open or infinite reading condemns the sensor.
6.Backprobe the harness with the key on, engine off. You should see battery voltage on the heater supply pin. If it's missing, work upstream to find the open.
7.If the heater is good and the signal wire has continuity, swap-test the sensor with the bank-2 downstream sensor (if same part number) to confirm the diagnosis before buying parts.

Repair cost

$5 – $500

Blown heater fuse is the cheapest possible fix — under $5 for the fuse. Wiring repair is highly variable: a single damaged pin is 30 minutes, a melted harness section is $200-500. Downstream O2 sensor replacement runs $150-400 depending on access. If the underlying problem turns out to be a failed catalyst that's contaminated the sensor, costs jump to $400-2,500, but that's a less common outcome here than with P0136.

Estimate your repair

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.