OBD Readiness And Freeze Frame Guide
Understand the readiness monitors your scanner shows and read the freeze frame data saved with a fault code.
Tool
Readiness monitors
Tick the monitors your scanner shows as “not ready” or “incomplete”.
Usually needs a steady drive of 10–15 minutes at constant speed on a faster road after the engine is warm.
Only some cars have one; it completes after a cold start followed by normal driving.
Often needs the car to stand overnight and a fuel tank between about a quarter and three quarters full; a cold start the next morning then runs the test.
Runs after a cold start; leave the car to cool down fully and start it the next morning.
Usually completes during steady driving after warm-up, with some idling at the end.
Usually completes within the first minutes after a cold start.
Often needs decelerations from a steady speed without braking.
If these are not ready, the fault memory was probably just cleared or the battery was disconnected.
Often need longer drives at higher load; the particulate filter monitor may need a completed regeneration.
Freeze frame data
When a fault code is stored, the control unit saves a snapshot of the main sensor values at that moment. It shows the conditions in which the fault happened.
| Value | How to read it |
|---|---|
| DTC that stored the frame | The fault code that triggered the snapshot. Read the other values with this code in mind. |
| Fuel system status | Open loop: the engine was cold or under full load and ignored the oxygen sensor. Closed loop: normal running with the sensor. |
| Calculated engine load | How hard the engine was working, in %. High values point to acceleration or towing; very low to idling. |
| Coolant temperature | Tells whether the engine was cold or warm. Many faults appear only cold or only hot. |
| Short and long term fuel trim | How much the control unit corrected the mixture. Large positive values mean it added fuel (lean mixture), large negative values that it removed fuel (rich). |
| Intake manifold pressure | Low at idle, near atmospheric pressure under full load on a non-turbo engine. |
| Engine speed (rpm) | Shows whether the fault happened at idle, in normal driving or at high revs. |
| Vehicle speed | Standing, town or motorway speed when the fault was stored. |
| Intake air temperature | Compare with the coolant temperature: both close to the outside temperature means a cold start. |
| Throttle position | How far the throttle was open, in %. |
How to use it
Tick the monitors your scanner shows as not ready (incomplete) to see what usually completes them. Below, the freeze frame table explains the values saved at the moment a fault code was set.
How it is calculated
Readiness monitors and freeze frame data are defined by the OBD-II standards (SAE J1979). For U.S. emission inspections the EPA guidance allows up to two monitors not ready on 1996–2000 petrol cars and one on 2001 and later cars; states and other countries can set their own rules. Driving needed to complete a monitor differs by maker.