Clearing a fault code and handing the vehicle back is not a repair. A few days later the warning lamp returns, or the vehicle fails its emissions inspection with “monitors not ready”. Both problems become easier with two often-ignored scan-tool functions: readiness status and the freeze-frame record. This guide shows how to read them and use them in the workshop on passenger cars, vans, trucks, tractors and motorcycles.
Monitors: the ECU’s own self-tests
Emissions-related OBD makes the engine ECU test its own systems. Every test group is called a monitor, and its status is reported as complete, not complete or not supported. Inspection programmes (the “I/M” in I/M readiness) check this status to see if the self-tests have actually been carried out.
Continuous group
Three monitors run constantly while their basic conditions are met: misfire (crankshaft acceleration per cylinder), fuel system (are fuel trims still inside their limits?) and comprehensive components (circuit and plausibility checks on sensors and actuators).
Non-continuous group
These run once per trip under defined conditions.
- Spark ignition: catalyst, heated catalyst, EVAP, secondary air, O2 sensor, O2 sensor heater, EGR/VVT.
- Compression ignition: NMHC catalyst, NOx/SCR aftertreatment, boost pressure, exhaust gas sensor, PM filter, EGR/VVT.
Evaporative system testing has its own EVAP diagnostics guide, so it is only touched on here as one of the monitors.
Enable conditions and the drive cycle
A monitor only starts when its enable conditions are true. Examples are engine temperature above a minimum, air temperature inside a window, a steady cruise for a set time, fuel tank level within limits for leak tests, and no related fault codes present. If one condition is never met, that monitor stays “not complete” no matter how far the vehicle is driven. A slow-warming engine (P0128) is a classic cause.
The drive cycle is the route that satisfies all of them: a cold start, idle, steady cruise and coasting phases. Each manufacturer has its own procedure, and some monitors wait for others to pass first.
Why readiness resets
Expect “not complete” after:
- clearing DTCs with a scan tool;
- a battery disconnect or ECU supply interruption on many models;
- ECU replacement or reflashing, whether with an update or with the original stock file.
“Not complete” is not a failure, but it also means nothing has been proven. This is why a car can fail inspection with no codes at all. Programme rules differ, and some allow one or two incomplete monitors. Some Ford models show manufacturer-specific code P1000 until testing is complete.
Pending, confirmed, permanent and the MIL
- Pending code (Mode $07): one failed test. No lamp yet on most monitors.
- Confirmed code (Mode $03): the failure repeated on a later trip. MIL on, freeze frame stored.
- Permanent code (Mode $0A, required on newer US-market vehicles): cannot be erased with a scan tool; it clears only after the ECU sees the monitor pass.
A steady MIL signals a confirmed emissions fault. A flashing MIL on a petrol engine warns of catalyst-damaging misfire. The lamp turns off after a run of passing trips, and the history is removed after a set number of warm-up cycles, depending on the regulation and the manufacturer.
Reading the freeze frame
Mode $02 stores a single snapshot when a code is confirmed, with priority usually given to misfire and fuel system faults. Typical contents:
- the DTC that stored it, fuel system status and calculated load;
- coolant temperature and short- and long-term fuel trims;
- rpm, vehicle speed and manifold pressure (MAP);
- often intake air temperature, throttle position and ignition advance.
Record it before clearing. Then use it to reproduce the fault: drive at the same rpm, load, speed and temperature while logging live data. A misfire frame stored at high load and low rpm points somewhere different from one stored at warm idle. A rich code (P0172) stored only when cold suggests a temperature input problem rather than a fuel delivery fault.
Mode $06: the numbers behind the tests
Mode $06 lists non-continuous test results with a measured value and minimum/maximum limits. On CAN-based vehicles each result is grouped by monitor ID and test ID with defined scaling. Look for tests that pass, but only just: a value close to its limit shows a system heading towards a code. Some test IDs are manufacturer-defined, so check service data.
Heavy-duty and agricultural: J1939 equivalents
Trucks, buses, tractors and combines typically use SAE J1939. DM1 lists active codes, DM2 previously active codes and DM5 readiness information. A code is an SPN (suspect parameter) with an FMI (failure mode) and an occurrence count, so SPN 110 FMI 4 means the coolant temperature signal is below normal or shorted low. Diesel aftertreatment monitors (SCR, DPF, NOx sensors) follow the same idea as on cars.
Motorcycles
From Euro 5, new EU motorcycles carry OBD stage II, which goes beyond circuit checks, and Euro 5+ adds requirements such as catalyst monitoring. Diagnostic connectors and generic mode support still differ between brands.
Workshop checklist
- Read all code types and readiness first, and save the freeze frame.
- Reproduce the fault from freeze-frame conditions while logging.
- Check Mode $06 margins and fuel trims.
- Repair the cause, then clear and complete the drive cycle.
- Recheck readiness and pending codes before returning the vehicle.
- Keep emissions hardware (catalyst, EGR, DPF, SCR/AdBlue) in place and working.
Before any ECU programming, save a full original read. Having the stock file means the ECU can always be returned to factory calibration and readiness re-run.
Related fault codes (DTCs)
Generic OBD-II/EOBD codes (SAE J2012) plus one manufacturer-specific and one J1939 example. Manufacturers add their own codes, so check service data for the exact vehicle.
| Code | Meaning | Typical causes |
|---|---|---|
| P0300 | Random/Multiple Cylinder Misfire Detected | Ignition, injector or fuel supply faults, vacuum leaks, low compression; set by the continuous misfire monitor |
| P0171 / P0172 | System Too Lean / Too Rich (Bank 1) | Air leaks, MAF/MAP errors, fuel pressure, leaking injectors, exhaust leak before the sensor; set by the fuel system monitor |
| P0128 | Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) | Thermostat stuck open, ECT sensor fault; a cold-running engine can also stop other monitors from running |
| P0133 | O2 Sensor Circuit Slow Response (Bank 1 Sensor 1) | Aged or contaminated upstream sensor, exhaust leak, wiring |
| P0135 | O2 Sensor Heater Circuit (Bank 1 Sensor 1) | Open heater element, blown fuse, wiring or connector fault |
| P0420 / P0430 | Catalyst System Efficiency Below Threshold (Bank 1 / Bank 2) | Worn catalyst, exhaust leaks, sensor faults, misfire history |
| P0440 / P0442 / P0455 / P0456 | EVAP System Malfunction / Small Leak / Large Leak / Very Small Leak | Loose or faulty filler cap, cracked hoses, purge or vent valve faults, canister damage |
| P0401 | Exhaust Gas Recirculation Flow Insufficient Detected | Carbon-blocked EGR passages or valve, sticking valve, vacuum or position sensor faults |
| P0411 | Secondary Air Injection System Incorrect Flow Detected | Failed air pump, stuck check or switching valve, blocked passages |
| P0299 | Turbocharger/Supercharger “A” Underboost | Boost leaks, sticking wastegate or VGT, actuator or sensor faults |
| P2002 | Diesel Particulate Filter Efficiency Below Threshold (Bank 1) | Damaged or cracked filter, pressure sensor or hose faults |
| P20EE | SCR NOx Catalyst Efficiency Below Threshold (Bank 1) | DEF/AdBlue quality or dosing faults, NOx sensor faults, aged SCR catalyst |
| P0606 | Control Module Processor | Internal ECU fault, supply or ground problems; check power and grounds before condemning the unit |
| U0100 | Lost Communication With ECM/PCM “A” | CAN wiring, connector, ECU power or ground faults, ECU not responding |
| P1000 (Ford, manufacturer-specific) | OBD Monitor Testing Not Complete | Not a fault: shown after codes were cleared or the battery was disconnected, until all monitors have run |
| SPN 110 FMI 3 / FMI 4 (SAE J1939) | Engine Coolant Temperature: voltage above normal or shorted high / below normal or shorted low | Trucks and machines: sensor circuit open or shorted, connector damage |
Closing
Save the freeze frame, reproduce the conditions, check Mode $06, and finish with a full drive cycle. That routine solves more comeback faults than any part swap.
Need the original stock file for a specific ECU hardware and software number? Browse the ECUFlashFiles original file catalogue.