Heater element inside the oxygen sensor has failed (open, internal short, or high resistance). Wiring or connector problem in the heater circuit: open, short to ground, or high resistance. Blown heater fuse or a power-supply problem to the sensor (check your service manual for the circuit). Less common causes listed by RepairPal: exhaust or intake air leaks, low fuel pressure, a faulty coolant temperature sensor, PCM software needing an update, or a faulty PCM.

Replace the Bank 1 Sensor 1 oxygen sensor if its heater is open or shorted (an internal fault can't be repaired). Repair damaged wiring or connectors in the heater circuit. Replace a blown fuse, and find out why it blew. Update PCM software if a bulletin applies.

P0135🌫 Emissions / oxygen sensorsDrive with caution, fix soon

P0135: O2 Sensor Heater Circuit (Bank 1, Sensor 1) (Meaning, Causes, Fixes)

Q: What does P0135 mean?

P0135 means the engine computer has decided the heater inside the front (upstream) oxygen sensor on Bank 1 isn't working. A failed heater element in the sensor is the most common cause, but wiring, connectors or a blown fuse can do it too. The upstream oxygen sensor (Bank 1, Sensor 1) sits before the catalytic converter on the side of the engine with cylinder #1 (on a 4-cylinder there's only one bank). It only reads accurately once it's hot, so it has a built-in electric heater to bring it up to temperature quickly and let the engine enter closed-loop fuel control sooner. OBD-Codes.com explains that the engine computer times how long the sensor takes to start switching; if too much time passes (judged against coolant temperature), it sets P0135. RepairPal describes the trigger as the PCM detecting the heater isn't operating, or the sensor voltage staying below about 400 millivolts for more than two minutes (or an air-fuel ratio sensor staying lean-biased too long), with exact criteria varying by make and model. P0155 is the same fault on Bank 2, and P0141 is the downstream (Sensor 2) heater on Bank 1.

Q: Can I keep driving with P0135?

Drive with caution, fix soon. Usually drivable, and often the only thing you'll notice is the check engine light and somewhat worse fuel economy. RepairPal rates it Moderate: not an immediate safety threat, but get it diagnosed within a reasonable time. Until the upstream sensor warms up and works, the engine can't fine-tune its fuel mixture, and a car with this code will likely fail an OBD2 emissions inspection.

Reviewed 2026-10-22 · Based on 2 cited source(s)

What P0135 means

The upstream oxygen sensor (Bank 1, Sensor 1) sits before the catalytic converter on the side of the engine with cylinder #1 (on a 4-cylinder there's only one bank). It only reads accurately once it's hot, so it has a built-in electric heater to bring it up to temperature quickly and let the engine enter closed-loop fuel control sooner. OBD-Codes.com explains that the engine computer times how long the sensor takes to start switching; if too much time passes (judged against coolant temperature), it sets P0135. RepairPal describes the trigger as the PCM detecting the heater isn't operating, or the sensor voltage staying below about 400 millivolts for more than two minutes (or an air-fuel ratio sensor staying lean-biased too long), with exact criteria varying by make and model. P0155 is the same fault on Bank 2, and P0141 is the downstream (Sensor 2) heater on Bank 1.

Severity: can I keep driving?

Drive with caution, fix soon

Usually drivable, and often the only thing you'll notice is the check engine light and somewhat worse fuel economy. RepairPal rates it Moderate: not an immediate safety threat, but get it diagnosed within a reasonable time. Until the upstream sensor warms up and works, the engine can't fine-tune its fuel mixture, and a car with this code will likely fail an OBD2 emissions inspection.

Common symptoms

Check engine light on (often the only symptom)
Poorer fuel economy (both OBD-Codes.com and RepairPal)
Rough idle or running (RepairPal)
Black smoke or bad-smelling exhaust in some cases (RepairPal)
In some cases nothing else is noticeable (RepairPal)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Heater element inside the oxygen sensor has failed (open, internal short, or high resistance)
Wiring or connector problem in the heater circuit: open, short to ground, or high resistance
Blown heater fuse or a power-supply problem to the sensor (check your service manual for the circuit)
Less common causes listed by RepairPal: exhaust or intake air leaks, low fuel pressure, a faulty coolant temperature sensor, PCM software needing an update, or a faulty PCM

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Read all stored codes. If P0135 shows up with other O2 heater codes (P0141, P0155, P0161) at once, suspect a shared fuse, relay or power feed before replacing sensors.
Find the Bank 1 Sensor 1 sensor (upstream, before the catalytic converter on the cylinder #1 side) and inspect its wiring and connector for melted insulation, chafing on the exhaust, corrosion or a loose plug.
Check the fuse that feeds the O2 sensor heaters (see your owner's or service manual).
With a multimeter, measure the heater resistance across the sensor's heater terminals and compare it to your vehicle's specification. An open circuit means the sensor needs replacing.
With the key on, check for battery voltage at the harness side of the heater circuit.
Ask a dealer, or search NHTSA, for technical service bulletins; RepairPal lists outdated PCM software as one possible cause.

Typical fixes

Replace the Bank 1 Sensor 1 oxygen sensor if its heater is open or shorted (an internal fault can't be repaired)
Repair damaged wiring or connectors in the heater circuit
Replace a blown fuse, and find out why it blew
Update PCM software if a bulletin applies

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

The code comes right back after a new sensor is fitted (forums on OBD-Codes.com are full of these cases, which usually point to wiring, power or the PCM)
The sensor is seized in the exhaust and won't come out without heat or special tools
You need circuit diagrams to trace the heater power and ground

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Do I just need a new oxygen sensor?

Often, yes: OBD-Codes.com's first-listed causes are a failed heater element inside the sensor, which can't be repaired. But wiring, connectors and fuses also cause P0135, so test the heater resistance and the circuit first. Many owners on OBD-Codes.com's forums replaced the sensor and still had the code.

Where is Bank 1 Sensor 1?

Sensor 1 is the upstream sensor, before the catalytic converter. Bank 1 is the side of the engine with cylinder #1; a 4-cylinder inline engine has only Bank 1.

Is P0135 serious?

Not usually an emergency. RepairPal rates it Moderate. The sources differ on symptoms: OBD-Codes.com mentions mainly poor fuel economy, while RepairPal also lists rough running, stalling and black smoke. Get it fixed reasonably soon.

Why do the sources describe the trigger differently?

OBD-Codes.com says the computer sets P0135 when the sensor takes too long to start switching; RepairPal says it sets when the heater isn't operating or the sensor stays below about 400 mV for over two minutes. Both note the exact test varies by manufacturer.

Sources

Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.