Warm-up Cycle 

It is accomplished after each startup when the engine coolant temperature (ECT) rises a minimum of 40°F (22° C) and reaches a minimum of 160°F (71° C). When 40 Warm-up cycles have been achieved, the ECM will clear all history diagnostic trouble codes (DTC) and freeze frame data (FFD). It does not clear permanent diagnostic trouble codes (PDTC).

Cold Soak 

The ECT and intake air temperature (IAT) sensors must be within approximately 10°F of each other and below 122°F (50° C) at startup. It is sometimes required to run the monitor tests for the air-fuel ratio sensor heater(s) (AFRS) and oxygen sensor heater(s) (HO2S), secondary air injection (AIR), and evaporative emission system (EVAP).

Enable Criteria

It is the operating conditions required to run each monitor test. Each monitor test will have its own enable criteria. If the enable criteria are not met, the monitor test cannot run. A failed monitor test result will set a pending or confirmed DTC.


It requires a key-on engine-off (KOEO) or key-on engine running (KOER) condition, completing the enable criteria for the monitor test, and then a key-off event. The trip ends once the scan tool no longer communicates with the ECM.

Readiness Flag Status

It Indicates if the ECM has completed testing of specific monitored emission systems. It does not indicate if the system passed or failed. If the monitor test required to flip the readiness flag to complete has not finished testing, the status on the scan tool will display Not Complete. If the monitor test needed to flip the readiness flag has completed testing, the readiness flag status will display Complete. A readiness flag showing Complete means the ECM has finished testing the system at least once since the memory was last reset.

Readiness Flag 

They are used to report that a specific emission-related system has completed testing. Each readiness flag can consist of one or multiple individual monitor tests. State emissions programs use some readiness flags as part of an official Inspection, and a minimum number of readiness flags must be Complete to pass an emissions Inspection.

Incomplete, Not Done, Not Ready

Since the last MODE $04 Diagnostic Clear was performed, one or more required readiness flag monitor tests have not run or have failed (Pre-CAN). Before the Controller Area Network (CAN) introduction in 2008, the PCM did not have to switch to Complete, Done, or Ready if a readiness flag monitor test failed.

Complete, Done, Ready

Since the last MODE $04 Diagnostic Clear was performed, all Complete readiness flag monitor tests have completed testing at least one time. With the introduction of CAN, the readiness flag status must switch to Complete, Done, or Ready once testing is complete, whether the monitor tests pass or fail.

Non-Readiness Mode

It occurs during normal operation when the enable criteria required to run a monitor test to set a readiness flag to complete is not met.  

Readiness Mode 

It occurs during normal operation when the enable criteria required to run a monitor test to set a readiness flag to complete is being met.  

Malfunction Indicator Lamp (MIL)

It is located on the dash and must be commanded "ON" when the ECM predicts tailpipe emissions exceeding 1.5x Federal Test Procedure standards.

Controller Area Network (CAN)

Powertrain modules communicating through DLC pins 6 & 14 use CAN ISO 15765 protocol. CAN protocol operates at 500kps and is installed on all 2008 and newer model vehicles.

Pre-Controller Area Network (CAN)

Powertrain modules do not communicate with the CAN ISO 15765 protocol. Typically, this is for model-year vehicles from 1996 through 2007. Protocols that might be used are SAE J1850 PWM, SAE J1850 VPM, ISO 1941, and ISO Keyword 14230.

CAN Phase-in

CAN "C" was phased starting in 2005 through the 2007 model year. It isn't easy to know exactly what vehicles were equipped with this protocol during these years. Be aware that these vehicles have CAN "C" and communicate to the scan tool through DLC pins 6 & 14.

Monitor Test

it is a system-specific test capable of setting a Diagnostic Trouble Code (DTC). Readiness Flags require one or more monitor tests to complete the testing of a component or system.

Continuous Monitor Tests

The ECM monitors misfires, fuel systems, and comprehensive components while their individual enable criteria are met.  A Continuous monitor test will run multiple times per trip so that it can pass and fail during the same trip. The readiness flag status related to misfire, fuel system, and comprehensive components is not used during a state emissions inspection.

Non-Continuous Monitor Tests 

When the enable criteria for a specific monitor test is met, the ECM will test at least once per trip if passing. The ECM will run multiple times per trip if a failing test result is indicated. Non-Continuous Readiness Flag Status is displayed in MODE $01. State Emissions Inspection programs use them as pass/fail criteria. They are all two-trip DTCs.