Mode Four clear codes is the OBD-II service a scan tool uses when you press erase or clear. It looks like a simple delete button, but the request removes much more than the trouble codes on the screen, and understanding its full reach prevents costly diagnostic mistakes.
This article examines the service itself: the data it erases, the data it cannot touch, and the conditions under which the engine computer accepts the command. Practical advice on when and how to clear codes after a repair is covered in a separate guide on clearing diagnostic trouble codes.
One Button, Many Consequences
In the SAE J1979 standard, service $04 is defined as the request to clear or reset emissions-related diagnostic information. When a scan tool sends it, every emissions-related control module that supports the service is expected to respond by resetting a whole group of stored data.
Many owners assume the command only switches off the malfunction indicator lamp. In reality, the lamp turning off is a side effect. The larger effect is that the computer forgets what it knew about recent faults and starts its emissions testing over from the beginning.
That reset has real value after a confirmed repair, because it gives the monitors a clean slate for verification. Used before diagnosis, though, it destroys evidence. Knowing exactly what disappears is the best protection against clearing at the wrong moment.
What Mode Four Erases
The standard specifies a broad list of information that a Mode 4 request resets. Not every vehicle stores every item, but a compliant emissions module clears all of the following that it supports.
- Confirmed trouble codes reported through Mode 3 and pending codes reported through Mode 7.
- Freeze frame data, the snapshot of sensor values saved when a code set.
- Readiness monitor status, which returns non-continuous monitors to incomplete.
- Oxygen sensor test results and on-board monitoring test results shown in Mode 6.
- The commanded state of the malfunction indicator lamp, which switches off.
- Distance and time counters linked to the lamp and to code clearing on vehicles that track them.
Each of these items helps a technician understand a fault. Freeze frame shows conditions at the moment of failure, Mode 6 shows how close tests were to their limits, and readiness shows what has been retested. Clearing removes all of it in a single step.
What Mode Four Cannot Touch
Some information is deliberately protected from the clear command. These protections exist so that clearing cannot be used to hide faults from inspectors, technicians, or buyers.
Permanent Codes
Permanent codes, reported through Mode 0A, survive a Mode 4 request. They remain until the engine computer itself runs the related monitor and sees it pass. This is why a car can show no stored codes after clearing yet still be refused at an emissions station that reads permanent codes.
The only legitimate way to remove a permanent code is to repair the fault and drive until the monitor completes and passes. No scan tool command, battery disconnection, or aftermarket device is intended to erase it.
Vehicle Information and Calibration
Mode 9 information, such as the vehicle identification number and the calibration identifiers for the installed software, is not affected. These values describe the vehicle and its programming rather than its fault history, so they stay intact after any clear request.
Inspection programs can compare these values to expected records, which is another reason clearing cannot disguise modifications or swapped computers.
Records Outside the Emissions System
Mode 4 targets emissions-related data. Codes held in airbag, ABS, body, or other non-emissions modules are generally not cleared by the generic request. Those modules use manufacturer-specific commands, which professional scan tools provide.
Some manufacturers also keep internal history or counters that generic tools cannot see. These records vary by brand and are not covered by the OBD-II standard.
Conditions Required for the Command to Work
The engine computer does not accept a clear request under every condition. The standard expects the ignition to be on, and many vehicles require the engine to be off, which prevents data from being reset while monitors are actively running.
If a scan tool reports that the clear request failed, check that the ignition is in the run position with the engine stopped and that the tool is still communicating. Some tools ask you to confirm the request because the action cannot be undone.
If codes return immediately after a successful clear, the fault is still present. Continuous monitors such as the comprehensive component monitor can detect circuit faults within seconds, so a code reappearing at once usually points to an electrical problem that still needs attention.
How Mode Four Affects Freeze Frame Evidence
Freeze frame data is often the most valuable casualty of clearing. It records values such as engine speed, coolant temperature, vehicle speed, load, and fuel trim at the moment a code set, offering a glimpse of the conditions that triggered the fault.
That snapshot can separate similar problems. A lean code set during cold idle suggests something different from the same code set at highway speed under load. Once Mode 4 runs, that context is lost, and the next occurrence may take days to capture.
Before any clear request, save a record that lets you continue diagnosis afterward. Many scan tools can export a report, but a photo of each screen works too. At minimum, capture the following.
- Every stored, pending, and permanent code, written exactly as displayed.
- The freeze frame values attached to each code.
- Readiness monitor status and the reported MIL state.
- The date, mileage, and any recent repairs or symptoms.
Effects on Mode Six and Monitor Data
Mode 6 reports the results of individual on-board monitor tests, including the measured value and the limits it was compared against. Technicians use it to see whether a component is passing comfortably or barely scraping by, which can reveal problems before a code sets.
A Mode 4 request resets those results along with readiness status. Until each monitor runs again, Mode 6 may show no data or default values. Reviewing Mode 6 before clearing preserves insight into marginal components, such as a catalyst that passed but only narrowly.
Mode 6 data is reported in standardized identifiers, but interpreting it often requires manufacturer information about which test is which. If you are not sure how to read it, save the data and let a qualified technician interpret it rather than guessing.
Why Clearing Is Not a Repair
Mode 4 changes stored information, not the vehicle’s physical condition. A vacuum leak, failing sensor, or misfiring cylinder remains exactly as it was. As soon as the relevant monitor runs, it will find the same fault and begin setting codes again.
Clearing can also make a situation worse. A flashing malfunction indicator lamp signals a severe misfire that can damage the catalytic converter. Turning off the lamp does not stop the misfire, and continued driving may turn an ignition repair into an exhaust system repair.
Never use Mode 4 to conceal a problem from an inspection station, a buyer, or a technician. Incomplete monitors and permanent codes reveal recent clearing, and hiding a known fault can carry legal consequences on top of the mechanical risk.
Generic Clear Versus Module-Specific Clear
Basic code readers send the generic Mode 4 request, which reaches emissions-related modules only. Professional scan tools can also send manufacturer-specific clear commands to individual modules, including airbag, ABS, and body controllers.
This difference explains a common surprise. After clearing with a basic tool, the check engine lamp may turn off while an ABS or airbag light stays on. Those lamps belong to other systems that ignore the generic request and need their own procedures.
Airbag system faults deserve particular caution. Never probe SRS wiring with a test light or ohmmeter, because accidental deployment can cause serious injury. Leave diagnosis and clearing of SRS codes to qualified technicians with the proper equipment and procedures.
Common Misunderstandings About Mode Four
One misunderstanding is that Mode 4 resets everything the computer has learned. Clearing affects diagnostic data, but whether adaptive values such as fuel trim learning are also reset depends on the manufacturer. Some vehicles have separate relearn or reset procedures for those values.
Another misunderstanding is that clearing improves performance. If the car runs better right after a clear, the improvement usually reflects a temporary change in learned values or simple coincidence, not a fixed fault. The problem remains until it is properly repaired.
A third misunderstanding is that clearing is invisible. Readiness monitors showing incomplete, freshly reset distance counters, and permanent codes all show that information was recently erased. Inspectors and experienced technicians check for exactly these signs.
Using Mode Four the Right Way
The right time for a Mode 4 request is after the fault has been diagnosed, the repair has been completed, and all codes and freeze frame data have been recorded. At that point, clearing turns the monitors into a tool for confirming the repair.
Afterward, expect readiness monitors to show incomplete. Plan several days of normal driving before an emissions inspection, then check readiness status and look for pending codes. A pending code that reappears is a sign the repair needs another look.
Treat the clear command with the same care you would give any irreversible action. Pause, confirm your notes are complete, and remember that the data you erase cannot be recovered. Used deliberately, Mode 4 becomes part of good diagnosis rather than a shortcut around it.