P0135 and your Nevada smog check

An oxygen sensor only reads accurately once it is hot. To get there quickly, most sensors carry a small electric heater. When the computer sees that heater's circuit misbehave on the first sensor of bank 1, it sets P0135. Here is what that means in plain words and how it plays out when you bring the car in for a smog check in Reno.

At a glance

• SAE J2012 name: O2 sensor heater circuit (bank 1, sensor 1).
• Sensor 1 sits before the catalytic converter and helps the computer set the fuel mixture.
• With the light on, a 1996 or newer light-duty vehicle fails in Nevada (DMV).
• Do not clear the code right before the test: the car comes back not ready.

Reading the code name

The generic SAE J2012 definition is "O2 sensor heater circuit (bank 1, sensor 1)." Each part of that name tells you something. O2 sensor is the oxygen sensor in the exhaust. Heater circuit is the electric element inside it and the wiring that feeds it. Bank 1 is the side of the engine with cylinder number one; an inline four-cylinder engine has only one bank. Sensor 1 is the upstream sensor, the one before the catalytic converter.

Upstream sensors matter a great deal. The computer reads them constantly to decide how much fuel to add, so a sensor that warms up slowly leaves the engine running on a rough estimate for longer after each start.

Why the heater exists

An oxygen sensor reports nothing useful while it is cold. On a winter morning in Reno, waiting for exhaust heat alone would take a while. The built-in heater brings the sensor up to working temperature quickly, so the computer can switch to accurate fuel control sooner and the converter can do its job.

The computer watches the heater's current or resistance. If it sees an open circuit, a short, or a value outside what it expects, it stores P0135 and lights the lamp. The fault is electrical; the sensor may still be reading oxygen once the exhaust heats it.

At the smog check

Nevada does not run a tailpipe probe on 1996 and newer light-duty vehicles. The analyzer plugs into the diagnostic port and reads the computer. A lit check engine light is an automatic fail, so P0135 fails the test even if the car feels the same as always.

The DMV still recommends testing. The failed report is a legal record of why the car failed, it helps with a reduced-cost ten-day movement permit if your registration has lapsed, and it is the first step toward a waiver if repairs get costly. Repairs paid for before that first failed test cannot count toward a waiver.

Common causes a repair shop checks

A repair shop treats P0135 as a starting point. Common causes a repair shop checks include a failed heater element inside the sensor, a blown fuse feeding the heater circuit, a damaged or chafed wire near the hot exhaust, a corroded or loose connector, and, less often, a problem with the computer's control of the circuit. The shop decides which applies; the code alone does not.

After the repair

A repair usually means the codes are cleared, and the DMV says cleared codes reset the readiness monitors. Plan on up to a week of normal driving, city and highway, before the retest so the computer can finish its self-checks. Testing too soon brings a rejection, which is not a failure but does mean a second trip.

If the car still fails after the work, Washoe's waiver path asks for receipts of at least $200 in related repairs at a 2G licensed authorized station, or at least $200 in related parts bought by the owner within 14 days after the initial test, then a visit to the DMV Emission Lab with both failed certificates. Our repair and retest page covers the order of steps.

The emissions warranty question

Before paying, the DMV suggests asking the dealer whether the vehicle is still under the federal emissions warranty or a recall. Federal law covers some expensive emission components for up to 8 years or 80,000 miles. Coverage depends on the part and the vehicle, so ask with your VIN in hand.