Mode 0A permanent DTCs: the codes you cannot clear

For years, the way to pass an emissions test with a fault was straightforward: clear the codes, drive to the test centre before the monitors completed, and hope.

Mode 0A exists to close that route.

Related: Mode 03 and 04 read and clear codes and clearing codes: when and why.

What a permanent code is

When a fault matures into a confirmed code and illuminates the warning lamp, the ECU stores it in two places: the normal code memory, and a separate permanent memory.

Clearing codes empties the first and does not touch the second. Neither does disconnecting the battery.

A permanent code is removed only when the ECU itself is satisfied the fault is gone — which means running the relevant monitor successfully, usually across more than one drive cycle.

Why that matters at a test

Most inspection regimes now check for permanent codes as well as current ones.

The scenario it defeats: a car with a live fault, codes cleared in the car park, presented with a clean scan. The monitors are incomplete, which is itself a fail in most regimes, but a permanent code makes it explicit.

OBD codes and MOT emissions test and OBD readiness monitors explained.

The practical consequence after a repair

This is where owners actually meet Mode 0A, and it causes confusion.

You fix the fault. You clear the codes. The lamp goes out. And the permanent code is still there.

That is correct behaviour. The ECU has not yet had the opportunity to verify the repair, because the relevant monitor has not run.

What clears it: driving normally until the monitor for that fault runs and passes, typically over one to three drive cycles depending on the system. EVAP monitors are the slowest, with narrow conditions on fuel level and temperature.

OBD drive cycle: how to complete and how the EVAP monitor works.

Do not clear codes before a test

Worth stating as a rule, because it is the most common self-inflicted failure.

Clearing resets every readiness monitor to “not ready”. A vehicle presented with incomplete monitors fails on that basis alone, regardless of whether it has any actual fault.

Fix, clear, then drive for several days, and check readiness before booking the test.

Buying a used car

This is the most useful application for a private buyer.

Read permanent codes on any car you are considering. They survive the seller’s pre-sale scan-and-clear, and they tell you what the vehicle was actually doing recently.

A car showing no current codes, no lamp, and a permanent code for a catalyst fault is a car whose codes were cleared shortly before you arrived.

Combined with readiness monitors, this is a strong two-part check:

Finding	Interpretation
No codes, monitors complete	Genuinely clean
No codes, monitors incomplete	Cleared recently
No codes, permanent code stored	Cleared recently, and here is what it was
Codes present	Read them properly

Used car pre-purchase OBD check and pending vs stored codes.

Support varies

Mode 0A was added to the standard later than the original modes, so:

• Vehicles built before roughly 2010 frequently do not support it.
• Support depends on the manufacturer’s implementation, not just the model year.
• Some tools do not request it, so a tool showing nothing is not proof of nothing.

If your vehicle returns no response to Mode 0A, that is a capability limit rather than a clean bill of health.

Supported PIDs and why parameters are missing.

The honest summary

Permanent codes are not an obstacle to work around. They are a verification mechanism, and they work in the owner’s favour more often than against:

• They stop a seller hiding a fault from you.
• They confirm your own repair actually worked.
• They prevent a test result that means nothing.

Read permanent codes alongside stored and pending ones — an ELM327 adapter and the free tier of the app are all it takes. On a used car, the permanent list is the one the seller could not edit.