P0135 Code: O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)

🟢 Low severity · drivable

Safe to drive. This affects only the sensor's warm-up heater, not drivability.

What P0135 means

The heater inside the bank 1 upstream oxygen sensor isn't working. Modern sensors have a built-in heater that brings them up to temperature quickly after a cold start; with a dead heater, the sensor just takes longer to start reading accurately. This is an electrical fault in the sensor itself or its heater circuit.

How the system works

Bank 1 is the side of the engine with cylinder 1, and sensor 1 is the oxygen sensor before the catalytic converter. That sensor only reads accurately once it is hot, so it has a small electric heater built in to get it up to temperature within a minute or so of a cold start.

The engine computer monitors how much current the heater draws. P0135 sets when the current is too low, too high, or absent. The sensing part of the sensor may still work, but until the exhaust heats it the engine runs on preset fuelling, which costs a little fuel economy and raises cold-start emissions.

Symptoms

Check engine light on
Usually no noticeable driving symptoms
Possible slight rich running when cold

Common causes

most common: Faulty oxygen sensor (heater element failed)
common: Blown fuse or damaged heater-circuit wiring
less common: Corroded sensor connector

How to diagnose P0135

Check the fuse first. The heater circuits usually share a fuse. If several heater codes appear together (for example P0135 with P0141), a blown fuse or a shared power feed is far more likely than several failed sensors.
Measure heater resistance. Unplug the sensor and measure across the two heater pins, which are normally the two wires of the same colour. A healthy heater reads a few ohms up to around 20. An open circuit means the heater element has failed.
Check for power at the connector. On the vehicle side of the connector, look for battery voltage on the heater feed with the engine running. No voltage means a fuse, relay or wiring fault.
Check the control side. The computer switches the heater's ground. If power is present and the heater tests good, check that control wire for an open or a short before suspecting the computer.
Inspect the harness near the exhaust. Sensor wiring that has dropped onto the exhaust pipe melts through quickly. Look for bare copper and repair or re-route it.

How to fix it

Fix	DIY?	Typical cost
Check the O2 sensor heater fuse and wiring	Yes	$0–20
Replace the bank 1 upstream oxygen sensor	Yes	$60–200

Common misdiagnoses

Replacing the sensor when the fuse is blown. The new sensor will set the same code straight away.
Fitting the wrong sensor. Upstream and downstream sensors, and bank 1 and bank 2, are often different part numbers with different connectors.
Cutting and splicing a universal sensor poorly. Heater circuits carry real current, so a bad joint brings the code back.

P0135 vs similar codes

P0141: P0141 is the same heater fault on the sensor after the catalytic converter on bank 1.
P0155: P0155 is the same fault on the upstream sensor of bank 2, the other side of a V engine.
P0130: P0130 concerns the sensor's signal circuit, not its heater. The reading itself is wrong or missing.

Frequently asked questions

Is P0135 serious?

No. It only affects how fast the sensor warms up, so drivability is unaffected. It may fail an emissions test until fixed.

What causes P0135?

A failed heater element inside the oxygen sensor, or a blown fuse in the heater circuit.

How much does it cost to fix P0135?

Typically $60–$200 for a new oxygen sensor.

Which side of the engine is bank 1?

Bank 1 is the side that contains cylinder 1. On a four-cylinder engine there is only one bank. On a V6 or V8 it varies by manufacturer, so check a firing-order diagram for your engine.

Will P0135 hurt fuel economy?

Slightly, and mainly on short trips. The engine stays on preset fuelling longer after a cold start because the sensor takes longer to warm up.