A catalyst monitor is a built-in diagnostic routine in your car’s computer that checks whether the catalytic converter is doing its job of reducing harmful exhaust emissions. It’s part of the OBD-II system found in all vehicles sold in the U.S. since 1996. If you’re seeing the term, it’s likely because a scan tool or emissions test showed your catalyst monitor as “not ready” or “incomplete,” meaning the test hasn’t finished running yet.
How the Catalyst Monitor Works
Your exhaust system has two oxygen sensors: one before the catalytic converter (upstream) and one after it (downstream). The catalyst monitor compares the signals from both sensors to evaluate converter performance. A healthy catalytic converter stores and releases oxygen as it cleans exhaust gases, and the two sensors reflect that process in distinct ways.
The upstream sensor shows rapid, frequent voltage fluctuations because it’s reading raw exhaust that constantly shifts between rich and lean. The downstream sensor, sitting after the converter, should show much smoother, slower fluctuations because the converter is absorbing and buffering those oxygen swings. When the car’s computer sees that stable downstream pattern, it knows the converter is working.
When a catalytic converter starts failing, it loses the ability to store oxygen effectively. The downstream sensor’s signal begins mimicking the upstream sensor’s rapid fluctuations. The computer detects this similarity and flags a problem. If it sees the same fault across three consecutive drive cycles, it turns on the check engine light and stores a diagnostic trouble code, most commonly P0420 (catalyst system low efficiency).
What “Not Ready” Actually Means
The catalyst monitor doesn’t run constantly. It only activates under specific driving conditions set by the manufacturer, and it requires the engine to be at normal operating temperature in a stable cruising state. Until those conditions are met, the monitor sits in an “incomplete” or “not ready” status. This doesn’t mean anything is wrong with your car. It simply means the test hasn’t had a chance to run yet.
Several things reset the monitor back to “not ready”:
- Battery disconnection: Unplugging the battery clears all stored data, including monitor status, learned fuel maps, and stored codes. Everything starts from scratch.
- Code clearing: Using a scan tool to erase trouble codes also resets all monitors to zero, forcing them to re-run their tests from the beginning.
- Engine computer reprogramming: Performance tunes or aftermarket modifications that alter the engine control module can deactivate oxygen sensors or prevent the catalyst monitor from operating at all.
Less obvious causes include exhaust leaks, faulty ignition components, or even a broken rear defroster on some vehicles, since the car’s computer may prioritize other systems before allowing the catalyst monitor to run.
How to Get the Monitor to Complete
Each manufacturer specifies a particular driving pattern, sometimes called a “drive cycle,” that triggers the catalyst monitor. The requirements vary, but they typically involve a combination of idling, steady highway driving, and moderate-speed cruising.
As an example, one Chevrolet model requires idling for 5 minutes, driving above 55 mph for 5 minutes, holding a steady 40 mph for 2 minutes, then holding 55 mph for another 2 minutes. Other vehicles simply require 10 minutes of sustained highway driving at 55 mph. Your owner’s manual or a dealership can tell you the exact cycle for your car.
After a battery disconnect or code reset, a general approach that works for many vehicles is to let the engine warm up fully, then drive at highway speed for at least a few minutes, coast down to about 20 mph, and accelerate back to highway speed. Repeating this a few times over 5 to 10 minutes often gives the computer enough data to mark the monitor as complete.
Why It Matters for Emissions Testing
In states that require smog checks, a “not ready” catalyst monitor can prevent your vehicle from passing. The rules vary by state and model year. California’s Bureau of Automotive Repair, for example, allows gasoline vehicles from 2000 and newer to pass with only the evaporative system monitor incomplete. For 1996 to 1999 gas vehicles, one incomplete monitor of any type is allowed. Diesel vehicles from 1998 to 2006 must have zero incomplete monitors to pass.
If your catalyst monitor is the one showing incomplete and your vehicle is a 2000 or newer gas model in California, you won’t pass. You’ll need to complete the drive cycle before returning for the test. The best strategy is to do the required driving a day or two before your appointment, then verify the monitor status with an inexpensive OBD-II scanner (available for under $30) before heading to the testing station.
When the Monitor Flags a Real Problem
If the catalyst monitor completes and triggers a P0420 code, that’s a different situation. It means the downstream oxygen sensor detected that your catalytic converter isn’t cleaning exhaust gases effectively. The converter may be worn out from age and high mileage, contaminated by oil or coolant leaks into the exhaust, or physically damaged.
A failing converter doesn’t typically cause drivability issues right away. Your car may run fine but produce higher emissions. Over time, a severely degraded converter can reduce engine performance or cause a sulfur smell from the exhaust. Replacement costs vary widely depending on the vehicle, but the repair is straightforward at most shops. Some states require the converter to be replaced before the vehicle can pass its next emissions inspection.