Catalyst Monitor Not Ready: What to Check

Catalyst Monitor Not Ready says an applicable catalytic-converter monitoring self-test has not reported completion in the relevant OBD readiness window. It does not by itself say the catalytic converter has failed, that you need a new exhaust part, or that an emissions test will necessarily pass or fail. 1 2

The catalyst monitor is part of the engine computer's emissions self-diagnostics. Conditions required to run it vary by vehicle and engine software. Clearing trouble codes, disconnecting power or not meeting the required operating conditions can affect completion; an unresolved fault may also require investigation. Readiness is a statement about a test's status, not a component replacement instruction. 1 1

First, Distinguish the Two Questions

1. Is the catalyst monitor complete? Look in the scan tool's I/M readiness section, preferably in the since DTCs cleared mode, and verify that Catalyst is supported.
2. Are there fault codes or an active MIL? Those are separate observations. For example, a catalyst-related trouble code is not the same information as an incomplete catalyst monitor. Whether a fault is active, pending or permanent can change which diagnostic evidence matters. 2

If the Check Engine Light is on, or a confirmed fault code is reported, do not treat driving alone as a substitute for professional troubleshooting. If the light is off and a single monitor remains incomplete, the best next step may be to check the required operating conditions and log another trustworthy reading. Neither pattern identifies a failed part automatically.

After a Battery Change or Code Clear

A reset can leave multiple monitors incomplete; the vehicle will need opportunities to run its self-tests again. If the catalyst monitor remains incomplete across comparable readings, consider whether the readings use the same horizon and whether the scanner identified the monitor consistently. Manufacturer instructions may require conditions not captured by a generic drive cycle. 1 1 3

How the Tool Helps

Enter the Catalyst status exactly as displayed, any other incomplete monitors, ECU MIL state, whether DTCs were checked and any known reset. The assistant will keep observation and hypothesis separate. A later scan can be compared to the first, then exported as a clean handoff report for a technician.

Record your scanner observations.

When to Get a Mechanic Involved

If the Check Engine Light flashes, the vehicle has serious drivability problems or the scanner reports active faults, do not attempt an emissions drive cycle as a workaround. Even without a warning light, persistent incomplete status may require OEM documentation or technician testing. This website intentionally does not recommend replacing the converter based solely on readiness.

Related: Readiness basics → Read the guide; monitors not completing → Read the guide; codes and readiness → Read the guide.