P0300 and your Nevada smog check

Start with the lamp itself: is it steady or flashing? With most codes the answer only affects the test. With a misfire it affects whether you should be driving at all, because a severe misfire sends unburned fuel into the catalytic converter, and the DMV warns that can cause immediate, expensive damage.

At a glance

• SAE J2012 wording: Random or multiple cylinder misfire detected.
• A flashing check engine light is a severe malfunction; the DMV says slow down or stop driving.
• Do not drive in for a smog check with a flashing light; the car needs a repair shop first.
• A steady lamp from P0300 fails a 1996 or newer light-duty vehicle at the Nevada OBD test.

Flashing or steady

The DMV draws a clear line between the two. A steady lamp means a problem that affects emissions has been detected. A flashing lamp means a severe malfunction that can damage the catalytic converter, and the DMV's advice is to slow down or stop driving and get the vehicle to a repair technician right away. EPA's consumer guidance says the same thing in its own words: a blinking light signals a severe problem, such as a misfire that can damage the converter, so limit driving and avoid high speeds and heavy loads.

So if your lamp is flashing with P0300 stored, the smog check is not the next step. A repair shop is.

Reading the code

SAE J2012 defines P0300 as: Random or multiple cylinder misfire detected. A misfire is a cylinder that fails to burn its charge properly on a given stroke. The computer spots it by watching the crankshaft speed, which stutters for an instant when a cylinder does not do its share.

When the misfires all come from one cylinder, the computer usually sets a cylinder-specific code instead. P0300 is the general version: the misfires are spread out or move around, so the computer reports the pattern rather than a single cylinder.

What the test shows

With a steady lamp, the car can be tested, and the result will be a fail for a 1996 or newer car or light truck. The DMV still recommends testing in that situation, since the failed report records why the vehicle fails and starts the waiver path. Repair costs from before that report do not count toward a waiver.

Misfire detection is also one of the self-checks the computer runs continuously, and clearing the code just before a test does not help: the DMV rejects a vehicle as not ready when codes were cleared, and the other monitors have to run again too.

Common causes a repair shop checks

Xpress tests; the repair shop finds the cause, and for emissions repairs the DMV recommends a 2G licensed authorized station. Common causes a repair shop checks for random misfires include:

• Worn spark plugs or failing ignition coils.
• A lean mixture from a vacuum leak, often seen alongside P0171 or P0174.
• Low fuel pressure or dirty injectors.
• Low engine compression or a mechanical problem.

Protect the converter, and the money

Misfires and catalytic converters are linked. A converter damaged by unburned fuel can go on to set P0420, turning one repair into two. That is the practical reason behind the DMV's flashing-light warning.

Converters are among the expensive emission components federal law covers for up to 8 years or 80,000 miles on some vehicles. The DMV advises checking emissions warranty coverage or a recall with the dealer before paying for repairs. A vehicle that is eligible for warranty coverage cannot receive a waiver.

Retest and waiver

Once the misfire is repaired and the lamp is off, give the car up to a week of mixed driving so the self-checks can complete, then retest. If repairs do not clear the lamp, Washoe County's waiver needs receipts of at least $200 for related repairs at a 2G station, or at least $200 in related parts bought by the owner within 14 days after the initial test.