How often to run diagnostics: a schedule that makes sense, not "once a year just in case"

“How often should I run diagnostics” usually gets the answer “once a year”. Better than nothing, but the logic is wrong: once a year you learn about what has already happened.

The point of regular checking is different — catching faults while the lamp is still off.

The monthly minimum: five minutes

Once a month, any time, unrelated to events:

1. Pending codes (Mode 07). The whole reason for doing this. Faults recorded once and not yet confirmed: no lamp, nothing the driver notices. Mechanics in code types.
2. Voltage. 12.4–12.7 V overnight, 13.8–14.4 V running. The trend is visible weeks before a no-start.
3. Readiness monitor states. All ready is a fast overall “nothing found” verdict. Covered in its own article.

Five minutes, no cost, and it closes most of the surprises.

The quarterly review: twenty minutes

Every three months, add live data:

• Long-term fuel trim at idle and at 2500 RPM. Drift toward positive means a growing vacuum leak — see the lean mixture guide.
• Warm-up temperature. Thermostats fail gradually.
• Per-cylinder misfire counters. Zeros are good.
• Boost on turbo engines at full load against the published figure.

These parameters change slowly. Quarterly is enough to see the trend.

Seasonal checkpoints: twice a year

Before winter — battery, thermostat, trims, plugs. Full list in the winter checklist.

Before summer — cooling fan, radiator, coolant, air conditioning. List in the summer overheating guide.

These two do not substitute for each other: winter exposes entirely different faults from summer.

Events that make a check mandatory

Here the trigger is not the calendar but what happened.

Event	What to look at
A lamp came on	Codes and freeze frame before clearing
Refuelled at an unfamiliar station	Knock correction, misfires — see bad fuel
After any repair	That codes really went rather than being cleared — post-repair checklist
Battery replacement	Monitors, adaptations, angle sensor calibration
Before a long trip	The full set — pre-trip checklist
Before an emissions test	Readiness monitors — drive cycle
Buying a car	The full protocol — used car inspection
The car behaves differently	Codes plus live data during the symptom

That last row matters most. Any change in behaviour is a reason to connect the same day, while the event is fresh and the freeze frame is still there.

Why “once a year at the workshop” is not enough

Not because workshops are bad. Because a workshop scan is a snapshot of one day.

Faults that develop gradually — an ageing oxygen sensor, a growing vacuum leak, a weakening battery — travel the whole distance from “nothing” to “broken” between annual visits. A yearly snapshot catches them by luck.

A monthly check at home catches the trend, which is entirely different information.

What not to do regularly

• Clear codes “to keep it clean”. The freeze frame goes with them and the monitors reset — see warranty and scanners.
• Leave the adapter plugged in permanently. Some cheap adapters never sleep and flatten the battery.
• Hunt for codes that are not there. No codes is a normal result, not a reason to dig deeper.

The schedule in one list

• Monthly, 5 min: pending codes, voltage, monitors.
• Quarterly, 20 min: plus trims, temperature, misfires, boost.
• Twice a year: seasonal checklists.
• By event: lamp, repair, refuelling, long trip, purchase.

In short

• Annual catches what happened; monthly catches what is about to.
• Pending codes are the main reason to connect regularly at all.
• Voltage shows the trend weeks before failure.
• A change in how the car behaves is a same-day reason to connect.
• Clearing codes “for tidiness” loses data rather than creating order.

Download the app — pending codes, voltage, and monitors take a minute with an ELM327 adapter over Bluetooth SPP on Android.