P0135 means the engine control module found a fault in the heater circuit of the upstream oxygen sensor on bank 1 (bank 1 sensor 1). The heater family runs P0030 through P0064, with P0135 as the general heater circuit code for bank 1 sensor 1 on many manufacturers' lists, beside P0031 (heater control circuit low) and P0032 (high) for the same sensor. Bank 1 is the bank containing cylinder 1; sensor 1 is the upstream sensor before the catalytic converter, the one that controls the mixture.
Severity: low. Can you keep driving? Yes. The heater exists to bring the sensor to its working temperature, several hundred degrees, within seconds of a cold start. Without it the sensor still works once exhaust heat warms it, so the engine runs in open loop on that bank a little longer after each start, with a small economy and emissions penalty. The check engine light stays on and the vehicle fails an emissions inspection.
What the heater is and how it is tested
The heater is a resistive element of a few ohms inside the sensor, fed with battery voltage through a fuse or relay and switched to ground by a driver in the module. Volkswagen's specification books describe the monitor's parameters: the module watches heater duty cycle, computes the sensor's ceramic temperature from its internal resistance, and expects the heater to bring it to operating temperature within a time window. A heater that draws no current (open), too much (shorted), or does not heat the element in time fails the test.
Three consequences:
- The oxygen reading is not what the code is about. A sensor with a perfect signal and an open heater sets P0135; a sensor with a bad signal and a good heater sets the P0130 family instead.
- The circuit has three parts that fail: the heater, the feed, and the connector. The heater element fails open with age and thermal cycling. The fuse or relay on the feed fails, often taking several sensors' heaters with it. The connector, sitting near the exhaust and under the vehicle, corrodes or loses terminal tension.
- The threshold is a calibration, and at least one manufacturer has issued software for it.
What the manufacturers documented
Ram 2500 and 3500 2010 to 2012 with the 6.7 L Cummins built before October 4, 2012 (bulletin 25-004-12, December 2012): for any oxygen sensor code, stored, pending or active, the instruction is to replace the oxygen sensor connectors and run the verification test, and do not replace any parts prior to performing this bulletin. The related bulletin for the 2011 model year explains why: some transmission harnesses were built with tin-plated terminals in five cavities of the oxygen sensor module connector that should have been gold-plated, and the terminals are replaced based on the harness part number. A heater code caused by the wrong plating on a terminal.
Volkswagen Jetta 2019 with the 1.4 L (technical bulletin 01-21-01, January 2021): an engine control module update whose list of addressed faults includes P0135, alongside P0133 (sensor slow response), P2279, P2181, P0111 and others. On these vehicles a heater code was among the things the software fixed.
Causes, in the order they are found
An open heater element inside the sensor, the most common single cause on a sensor past 100,000 miles; infinite resistance across the heater pins.
A blown fuse or a failed relay on the heater feed, shared by several sensors: two or more heater codes at once mean the feed.
A connector corroded, burned by exhaust heat, or with lost terminal tension, as on the Ram trucks.
A heater wire chafed where the sensor pigtail passes the exhaust manifold or a bracket, shorted to ground or open.
A sensor contaminated so the element cannot heat correctly; less common.
A calibration flagging the heater, as on the VW.
The module driver, last.
A diagnostic order
- Record companion codes. P0135 alone means this sensor or its wiring; P0135 with P0141, P0155 or P0161 means a shared fuse or feed; P0135 with P0130 family codes means the sensor as a whole.
- Check for a bulletin. On the Ram Cummins trucks the connector comes first, by instruction.
- Check the heater fuse and relay for bank 1 sensor 1.
- Unplug the sensor and measure heater resistance across its two heater pins: a few ohms per the specification (typically 3 to 15 ohms cold). Infinite is open; near zero is shorted.
- Key on, sensor unplugged: measure the feed pin for battery voltage. None means the fuse, relay or feed wire.
- Check the control wire to the module for a short to ground and for continuity, with the module disconnected.
- Inspect the connector on both halves for corrosion, heat damage and terminal tension; repin or replace it if in doubt.
- Check for a software update if the heater, feed and wiring pass.
- Replace the sensor only if its heater measured open or shorted, using the correct part (heater resistance differs between sensors that look alike), clear the code, and let the readiness monitors complete before an inspection.
What P0135 does not tell you
It does not tell you the oxygen sensor is bad; it tells you its heater circuit failed a test. An open heater, a blown fuse, a corroded connector, a chafed wire and a calibration all fail it, and Chrysler's bulletin replaces the connectors before any sensor. The heater's resistance and the feed voltage, measured at the sensor's connector, decide in minutes; the general principle is covered in what a trouble code does not tell you.
Frequently asked questions
Can I drive with a P0135 code?
Yes. The sensor still works once exhaust heat warms it; the engine runs slightly longer in open loop after a cold start. Expect a small economy penalty and a check engine light, and a failed emissions test until it is fixed.
Is P0135 the oxygen sensor?
Often, because the heater inside the sensor is the part that fails most. But a blown heater fuse, a shared relay, a corroded connector or a chafed wire sets the same code, and on 2010 to 2012 Ram 2500 and 3500 Cummins trucks Chrysler's instruction is to replace the oxygen sensor connectors before any sensor. Measure the heater's resistance and check the fuse first.
What is the difference between P0135 and P0130?
P0130 through P0134 concern the sensor's oxygen signal: its voltage, response and range. P0135 concerns the heater circuit inside the same sensor. A sensor can fail one and pass the other.
Why did two heater codes set at once?
Because the heaters on several sensors usually share a fuse or a relay. P0135 with P0141 or P0155 points at the shared feed, not at two sensors failing on the same day.
How much does it cost to fix P0135?
A fuse costs almost nothing. A connector repair is under an hour. An upstream oxygen sensor is a moderate part with a short replacement when it comes out cleanly; seized sensors add time. A software update is an hour at a dealer.
Related codes
- P0031 / P0032: bank 1 sensor 1 heater control circuit low and high.
- P0141: bank 1 sensor 2 heater; P0155 / P0161: bank 2 heaters. Several at once mean a shared feed.
- P0130 to P0134: bank 1 sensor 1 signal circuit, the sensing side.
- P0051: bank 2 sensor 1 heater control circuit low; see P0051.
- P0420: catalyst efficiency, whose monitor depends on the same upstream sensor; see P0420.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.