P0172 and your Nevada smog check

Rich is the opposite of lean: too much fuel for the air coming in. Drivers sometimes notice it as a fuel smell, black soot at the tailpipe or a drop in mileage, and sometimes notice nothing. A rich engine is hard on the catalytic converter over time, which is one more reason to handle P0172 before it turns into a second, more expensive code.

At a glance

• The SAE J2012 definition is System too rich (bank 1).
• Rich means the computer cannot trim enough fuel to balance the mixture on bank 1.
• A lit lamp fails the Nevada OBD test; a rich condition left alone can also wear on the converter.
• Gasoline vehicles may not give off visible smoke anywhere in Nevada, according to the DMV.

What rich means

SAE J2012 is the standard that fixes the meaning of generic OBD II codes. Its definition of P0172 is: System too rich (bank 1). The engine computer reads the front oxygen sensor and keeps trimming fuel to hold the mixture steady. When it has trimmed fuel as far as its limit and bank 1 still reads rich, it stores P0172.

On a four-cylinder engine bank 1 is the whole engine. On a V engine it is the side with cylinder 1, and the lean codes for the two sides are P0171 and P0174.

Why the computer cares

Unburned fuel is exactly what emission controls are meant to keep out of the air. Extra fuel also reaches the catalytic converter, which has to burn it off, and over time that heat and residue can shorten the converter's life. A converter that falls behind sets its own code, P0420, so a rich condition is worth dealing with early.

One rule at the smog check

A 1996 or newer car or light truck with its check engine lamp on fails Nevada's OBD inspection. That is the whole rule, and it applies to P0172 the same as any code.

Two related DMV points are worth knowing. First, the failed report is not wasted: it is the legal record that opens the waiver path and supports a ten-day movement permit at reduced cost if the registration has expired. Second, gasoline vehicles must not emit visible smoke anywhere in Nevada, and a smoking vehicle can be cited or ordered to test; a smoking vehicle is also not eligible for a waiver.

Common causes a repair shop checks

Xpress Smog Check Center tests vehicles; diagnosing a rich mixture is a repair shop's job, and the DMV recommends a 2G licensed authorized station for emissions repairs. Common causes a repair shop checks include:

• A leaking or sticking fuel injector.
• Fuel pressure that runs too high.
• An air flow sensor that reads high, or a badly restricted air filter.
• A front oxygen sensor giving a false rich reading.
• A purge valve letting fuel vapor into the engine at the wrong time.

What to do before you come in

Look at the timing. A passing test is good for 90 days, and testing early in your renewal window leaves time for a repair. If the car has just been repaired, wait until it has been driven up to a week in mixed conditions; clearing codes right before a test gets the vehicle rejected as not ready.

If the repair involves an expensive emission component, call the dealer before paying to ask about federal emissions warranty coverage or a recall, which can cover some components up to 8 years or 80,000 miles.

Pull any insurance tracker or monitoring device out of the diagnostic port before you arrive. The DMV requires technicians to fail a vehicle with one plugged in.

After a failed test

The order in Washoe County is fail, repair, retest. If the lamp is still lit after repairs, the vehicle may qualify for a waiver with receipts showing at least $200 in related parts and labor at a 2G station, or at least $200 in related parts the owner bought within 14 days after the initial test. Repairs before the first failed test do not count. The repair and retest page has the next steps.