Catalyst Monitor Not Ready? How to Get It Ready and Pass Your Emissions Test
A catalyst monitor that reads "not ready" or "incomplete" means the engine computer hasn't finished grading your catalytic converter since its memory was last reset. In most cases nothing is broken: the codes were cleared, the battery was disconnected, or your daily driving never meets the test's conditions. A specific drive pattern usually fixes it. If it doesn't, the monitor's own data points to what is blocking it.
What the Catalyst Monitor Checks
The catalyst monitor is the engine computer's self-test of whether your catalytic converter can still store oxygen, which is part of how it cleans exhaust. It's a non-continuous monitor: it runs only when the engine computer sees the right conditions, and until it finishes, your scanner shows "not ready," "incomplete" or "INC." All three mean the same thing.
Catalyst Monitor Sensors: Upstream vs Downstream
| Sensor | Location | Healthy signal during the test |
|---|---|---|
| Upstream O2 or air-fuel sensor | Before the converter | Moves constantly as the engine adjusts the mixture |
| Downstream O2 sensor | After the converter | Stays fairly steady, because the converter absorbs the swings |
There's no separate "catalyst monitor sensor." The test uses the same oxygen sensors that control fuel trim, which is why a lazy or failing sensor can keep the monitor from finishing.
What Happens During the Catalyst Monitor Test
Once the engine is fully warm, in closed loop and cruising steadily, the computer compares the two sensor signals. Downstream signal mimics upstream → the converter can't store oxygen. One failed test usually sets a pending P0420 (bank 1) or P0430 (bank 2); a second failure on a later trip turns on the check engine light.
An exhaust leak ahead of the rear sensor lets outside air skew that comparison, so listen for ticking at the manifold or flex pipe.
Why the Catalyst Monitor Is Not Ready
Usually because something reset the monitor's memory, or your driving never meets its test conditions:
- Codes were cleared recently (most common). A scanner's clear function wipes all completed monitors, the catalyst included.
- The battery was disconnected or replaced (common after repairs). Many vehicles lose readiness with power.
- Short trips only (very common). The engine never stays warm and steady long enough.
- A prerequisite hasn't passed yet (check if driving doesn't help). Many vehicles run the catalyst test only after the O2 sensor monitors pass.
Catalyst Monitor Not Ready but No Trouble Codes
No codes and plenty of highway miles, yet still incomplete? The usual causes don't set codes on their own:
- Thermostat stuck partly open: coolant never reaches the temperature the test needs.
- Slow upstream sensor: it passes its own check, but the computer won't trust it to grade the converter.
- Software issue: some makers publish bulletins with special readiness procedures or updates.
Watch coolant temperature and O2 sensor activity in live data during a drive, and if your scanner shows Mode 6 results, look for tests sitting close to their limits.
How to Fix Catalyst Monitor Incomplete in Five Steps
Rule out a pending code first, then confirm the engine warms up, drive the factory pattern and rescan after each attempt:
- Check codes and readiness together. Pending codes count. A pending P0420 means the test ran and failed, so driving more won't help.
- Ask what happened last week. A shop visit, a new battery or a jump start often explains everything.
- Confirm the engine gets warm. Coolant should reach normal operating temperature on the gauge or in live data.
-
Drive the factory pattern. Dealers can print it, and some makers publish it as a service bulletin. On most gas cars it comes down to a steady cruise with a fully warm engine, but the details vary by vehicle, for example:
- VW Tech Tip 01-19-03TT (2013-2018 Jetta, Golf, Beetle, Passat): accelerate smoothly to 50-55 mph and hold it on cruise for 5 minutes or more.
- BMW SI B12 14 18 (335d, X5 xDrive35d diesel): after an overnight cold start, drive at about 23 mph without stopping until the transmission shifts to 3rd.
- Rescan after each attempt. "Ready" with no code means the converter passed. A pending P0420 (bank 1) or P0430 (bank 2) means the test did finish but the converter failed it; that's a different problem from "not ready," so more driving won't clear it, and the next step is checking the converter, the oxygen sensors and the exhaust for leaks. Two or three correct patterns with no change at all → the cause is a sensor or a prerequisite, not your route.
Red Flag: If the check engine light comes on during your drive pattern, stop the pattern and read the code. Readiness won't matter until that fault is repaired.
How Long to Drive to Get a Catalyst Monitor Ready
Our estimate: with the correct pattern, the catalyst portion often takes 5 to 20 minutes of driving after warm-up. With ordinary mixed driving, plan on a few days to a week, including at least one longer highway trip. Rule of thumb: one correct highway pattern does more than a week of errands.
Can You Pass Inspection With the Catalyst Monitor Not Ready?
Often, yes, as long as the check engine light is off and you're within your state's allowance for unready monitors. Texas DPS, for example, passes 2001 and newer vehicles with one non-continuous monitor not ready, and 1996-2000 vehicles with two. Your state's inspection program sets its own limit, and it may differ.
- If the catalyst is your only incomplete monitor, you may pass where one is allowed.
- If EVAP is also incomplete, you're likely over the limit on a 2001+ car.
- A lit check engine light usually fails regardless of readiness.
How to pass a catalyst monitor check with the least risk: confirm readiness with a scanner the day before inspection, and if the catalyst still reads incomplete, make sure it's the only one.
LAUNCH Scanner for Catalyst Monitor Readiness
The LAUNCH X431 CRP919X covers every check in this guide with one tool. Here's which of its functions answers which catalyst monitor question:
| What you need to know | CRP919X function |
|---|---|
| Did last night's drive finish the catalyst monitor? | I/M readiness status for every emissions monitor |
| Is a pending P0420 or P0430 waiting behind the light? | Reads pending and stored codes, with freeze frame data |
| Why won't the monitor start? | O2 sensor diagnosis, plus a multi-stream live data graph of the upstream and downstream sensors and coolant temperature |
The CRP919X is an all-system 7-inch tablet, so the same tool handles the rest of the car too. Get more savings on it in our current LAUNCH deals.
Summary
A catalyst monitor that won't finish is usually a timing problem, not a parts problem. Check codes and reset history, drive your vehicle's own pattern, and confirm readiness before inspection day. If it still won't set, live data will show why.
FAQ
Is "catalyst monitor not ready" the same as a P0420 code?
No. Not ready means the test hasn't finished. P0420 means it finished and the converter failed.
Does a new catalytic converter reset the monitor?
The codes cleared after the install reset it. Plan a drive pattern before inspection; our catalytic converter replacement guide covers the install side.
Why did my catalyst monitor go back to not ready?
Something reset it: a code clear, a battery disconnect, or a module update during service.
Do hybrids take longer to complete the catalyst monitor?
Often, yes. The gas engine shuts off so frequently that the steady, warm running time the test needs is harder to get.
Can I pass with a "not supported" catalyst monitor?
Yes. "N/A" means your vehicle doesn't run that test, so inspection stations don't count it as incomplete.
