Lambda sensorsGeneric OBD-II codeUrgency: Garage soon
Code P0135: lambda sensor heater (bank 1, sensor 1)
P0135 means: oxygen sensor (lambda sensor) heater circuit malfunction, bank 1, sensor 1.[1][2][3] The engine ECU has detected an electrical fault in the circuit that heats the front lambda sensor on bank 1, the one upstream of the catalytic converter. That heater is a resistor built into the sensor that quickly brings it up to operating temperature after starting; with the sensor cold, its measurement of the oxygen in the exhaust gases is not reliable. Autodoc Club sums it up as a sensor that took too long to start working.[1][3][4][5]
What code P0135 means
The engine ECU has detected an electrical fault in the circuit that heats the front lambda sensor on bank 1, the one upstream of the catalytic converter. That heater is a resistor built into the sensor that quickly brings it up to operating temperature after starting; with the sensor cold, its measurement of the oxygen in the exhaust gases is not reliable. Autodoc Club sums it up as a sensor that took too long to start working.[1][3][4][5]
While the sensor is not hot, the ECU cannot rely on it to adjust the mixture and takes longer to switch to closed-loop control. That is why the effect is mainly noticeable when cold (slightly higher consumption and slightly uneven running until the engine warms up), and in many cases the only visible sign is the warning light. Samarins describes it as a purely electrical code, easy to diagnose with a multimeter.[1][2][4][5]
It is a generic OBD-II code (it starts with P0), so it means the same on any make. Some sources, such as Samarins, call the front sensor an "A/F sensor" (air-fuel ratio); it is the same part the code refers to.[4][6]
Original definition (SAE J2012): O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)[1][2][3]
Causes of P0135
From most to least likely, according to the sources.
- 1Faulty internal heater in the front sensor, with a resistance out of specification (for example, too high). On some Hondas, Lexuses and Toyotas it is a known cause.[1][2][4]
- 2Blown or missing sensor heater fuse.[2][4]
- 3Damaged, shorted or broken wiring between the sensor and the ECU (or the fuse box), or a connector with corrosion, damaged terminals or melted plastic.[1][2][4]
- 4Incorrect replacement sensor, or one of a part number that does not match the car.[4]
- 5Battery with a low charge.[4]
- 6Faulty engine ECU (ECM) or its wiring.[1]
Symptoms
- Engine warning light on; it often comes on after a cold start.[1][2][4]Engine warning light: faults that cause it
- Increased fuel consumption, especially on cold starts.[1][2]Uses more fuel: faults that cause it
- Slightly uneven engine running until it warms up and the sensor starts working.[1][2]
- In many cases nothing is noticeable other than the warning light.[4]
How to diagnose P0135
In order: from the most likely and cheapest to check to the most expensive.
- 1Read the code with a diagnostic tool, clear it and start again: if it comes straight back, the fault is permanent; if not, it may be intermittent. This is the first step of the workshop manual that Samarins follows in its example.[4]
- 2Check the fuse and, if the circuit has one, the sensor heater relay: it is the quickest and cheapest thing to check. The fuse box cover usually shows where each one is.[2][4]
- 3Inspect the sensor's connector and wiring: corrosion, damaged terminals, melted plastic, and chafed, shorted or cut wires between the sensor and the ECU or the fuse box.[1][2][4]
- 4Unplug the sensor and use a multimeter to measure the resistance of its heater: it should be low, a few ohms (Samarins gives a typical range of 0.9 to 10 Ω depending on the car), and match the value in the workshop manual for your model. A reading far off indicates a faulty heater: in Samarins's example, 235 kΩ against the 2.5-4 Ω specification.[2][4]
- 5Check that 12 V and earth reach the sensor connector and that the battery is well charged: a low charge and corroded wiring (even inside the fuse box) can trigger this code.[4]
- 6Replace the bank 1 front sensor only if the fuse, connector and wiring are fine and the heater resistance is out of specification. Check that the replacement is the correct part number for your car (using the VIN): an unsuitable sensor can also trigger the code.[2][4]
- 7If everything above is fine, the fault may be in the engine ECU or its wiring: a garage with the manufacturer's wiring diagrams should check it.[1]
Can I keep driving with P0135?
Garage soon
The sources agree that it is not a safety fault, although with some nuances: Carista classes it as low severity, Autodoc Club says you can drive for a while with the code active but recommends repairing it with moderate urgency to avoid bigger engine faults, and Samarins points out that often the only symptom is the warning light. It is best not to leave it for long: with the sensor unheated, mixture control gets worse and consumption rises.[1][2][4]
Documented faults related to P0135
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
Engine sensors: the most serious faults by engine and car
- High severityFrequency: Occasional2.0 DTI and 2.2 DTIVP44 injection pump and air mass meter failureGerman Wikipedia documents recurring failures of the Bosch VP44 injection pump and the air mass meter in the early years of this engine. An owner testimony for this engine describes the pump being replaced at 192,500 km, at a cost the owner calls «prohibitive».
- Medium severityFrequency: Common2.0 TDIClogged particulate filter or faulty pressure sensorThe 2.0 TDI's particulate filter clogs if the car mostly makes short journeys, because it never gets to regenerate; sometimes the fault is just the differential pressure sensor. Cleaning it costs around €200 and replacing it, €800 to €3,000.Repair (garage): €800–€3,000
- Medium severityFrequency: Common1.4 and 1.6 8vCrankshaft position sensor failure (TDC sensor)The crankshaft position sensor (capteur PMH, or TDC sensor) appears repeatedly in the reliability sources for the Clio II, the Kangoo I and the Dacia Logan I with this engine, with hard cold starting or stalling as the main symptoms. It is a cheap part, but the failure can leave you stranded.
- Medium severityFrequency: Common1.4 and 1.6 16vCrankshaft position sensor failureThe crankshaft position sensor (capteur PMH) appears repeatedly in the reliability sources for almost every generation with this engine, with hard cold starting, stalling or the car cutting out unexpectedly. It is a cheap part, but the failure can leave you stranded.
Wiring, connectors and short circuits: the most serious faults by engine and car
- Critical severityFrequency: OccasionalBMW N62 / N63Additional turbo coolant pump with a risk of short circuit and fireIn 2011 BMW recalled 5, 6 and 7 Series, X5 and X6 models built between 2007 and 2011 because of the additional turbo coolant pump: moisture, a short circuit and, in extreme cases, a fire. In 2012 there was another campaign for a crack in the pump, with a more robust version.
- High severityFrequency: Occasional1.3 TCeBlown fuel pump fuseOn the Renault Captur II and Clio V with the 1.3 TCe engine, a 20-amp fuse can blow and leave the car without an engine. According to L'argus, in April 2021 this fault still had no factory fix; the official service centre's diagnosis includes checking, and sometimes replacing, the catalytic converter.
- High severityFrequency: OccasionalM264/M254Poorly tightened 48 V earth wiring in the mild-hybrid system (fire risk)Mercedes-Benz has issued several recalls (2022-2025) because the 12 V and 48 V earth connections of some cars with a 48 V system may be poorly tightened: resistance rises, the connection heats up and a fire cannot be ruled out. The official workshop checks them and repairs them if necessary in an hour or more.
- High severityFrequency: Occasional1.9 TDIUnit injector tappet wear and injector wiring damageOn the unit injector system, the tappets that operate the injectors can wear if the oil is not changed on time, and the injector wiring can be damaged by contact with oil, causing misfires or loss of power.
- Critical severityFrequency: CommonRover 75Seat SRS connector fault with airbag and seatbelt warning light onAuto Express warns that the Rover 75's seat SRS connector can cause the airbags to fail, and dealers were fixing it with revised connectors. The Dutch APK inspection puts the airbag or seatbelt fault warning among the most frequent defects across almost all series.
- Critical severityFrequency: OccasionalAlfa Romeo GiuliettaShort circuit from badly routed wiring that can prevent gear shifting (KBA campaign, 2010-2015)Yes: according to Germany's KBA, on Alfa Romeo Giulietta cars built between 2010 and 2015 inadequate cable routing could cause a short circuit and, as a result, gear-shift failure.
Frequently asked questions
What is the difference between P0135 and P0130?
Both affect the same lambda sensor (the front one on bank 1, upstream of the catalytic converter), but different circuits. P0135 points to the sensor's heater circuit, the built-in resistor that brings it up to operating temperature after starting. P0130 points to the circuit of the sensor itself, the one that carries its oxygen measurement signal. That is why P0135 is diagnosed as an electrical problem of the heater: fuse, wiring and heater resistance.[2][3][4][5][7]
Can I drive with code P0135?
Yes, for a while: the sources consulted consider it a low-severity fault that does not compromise safety. Even so, it is best to repair it without leaving it long: with the sensor unheated, the ECU controls the mixture less well and consumption can rise, especially when cold.[1][2]
Does the lambda sensor have to be replaced when P0135 appears?
Not at first. It is an electrical code and is diagnosed with a multimeter: first the fuse (and the relay), the connector and the wiring, and then the resistance of the sensor's heater against the manual's value. The sensor is replaced only if that resistance is out of specification and everything else is fine, checking that the replacement is the correct one.[2][4]
Does P0135 mean the same on all makes?
Yes. Codes starting with P0 are generic OBD-II codes and have the same definition for all manufacturers. What changes from one car to another is the type of sensor, the resistance the heater should have and where the fuse is, details you need to look up in the workshop manual for each model.[4][6]
Will I pass the MOT with code P0135?
If the code has switched on the engine warning light, no: with that light on, the inspection records a serious defect, and on Euro 5 and Euro 6 cars the ITV (Spain's roadworthiness inspection, similar to the MOT) also reads the OBD diagnostic system. Repair the fault before you go; clearing the code just before the inspection does not help either, because it counts as a serious defect.[8]
Not sure what is wrong with your car? Describe what you notice in the symptom diagnosis or see all fault codes.
Sources
Every fact on this page links to its source. If you find an error, let us know.
- Código de avería P0135 - guía paso a paso para su diagnóstico y resoluciónAutodoc Club · Technical documentation · consulted 2 October 2026 · in Spanish
- P0135: O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)Carista · Technical documentation · consulted 2 October 2026
- P0135 — O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)The Automotive Technician (TaT) · Technical documentation · consulted 2 October 2026
- Code P0135 - Heated Oxygen Sensor Heater Circuit Malfunction (Bank1 Sensor1)Samarins.com · Technical documentation · published 19 January 2019 · consulted 2 October 2026
- Oxygen sensorWikipedia · Other source · consulted 2 October 2026
- On-board diagnosticsWikipedia · Other source · consulted 2 October 2026
- P0130 — O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)The Automotive Technician (TaT) · Technical documentation · consulted 2 October 2026
- Desmontando mitos: ¿puede pasar mi coche la ITV con el testigo de «avería motor» encendido?Diariomotor · Motoring press · published 8 March 2021 · consulted 2 October 2026 · in Spanish