Electrical
The charging system and the diagnostic data stream are where a PPI gets objective fast.
By Anthony LimpertASE Master Automobile Technician- Voltage check is fast and definitive. Engine off = surface charge (12.6V). Running at idle = 13.8-14.7V. Loaded (headlights + heater + rear defrost) = should hold above 13.5V. If it drops to battery voltage, alternator is weak.
- Modern testers (Midtronics, Solar) measure conductance and report CCA at ambient temperature. Compare measured CCA to the sticker rating. Below 75% of rated = replacement due. AGM batteries need AGM-mode.
- Current codes only tell part of the story. Pending codes = intermittent issues brewing. Readiness monitors reveal recent code clears (all not-ready = someone hid a problem). Long-term fuel trims show engine health.
- Battery disconnect or code clear resets monitors. If the seller says nothing was changed but monitors are not-ready, they may have cleared a CEL to pass inspection. Complete a drive cycle and rescan.
A multimeter, a battery tester, and an OBD-II scanner produce numbers no seller can argue with, and the scan data in particular is where you catch the sellers who tried to hide a problem the week before you arrived.
Battery and Charging System in Five Minutes
The alternator check is a three-reading voltage test at the battery. Engine off, a healthy battery with a surface charge reads about 12.6 volts. Engine running at idle, charging voltage should sit in the 13.8 to 14.7 volt range. Then load the system with headlights, heater blower, and rear defroster all on: voltage should hold above 13.5 volts. If loaded voltage sags down toward battery voltage, the alternator is weak and living on borrowed time.
For the battery itself, do not settle for a visual check or seeing whether the car starts. Use a conductance tester such as a Midtronics or Solar unit, or a carbon-pile load tester. These report measured cold cranking amps at the ambient temperature, which you compare to the CCA rating printed on the battery sticker. A battery testing below 75 percent of its rated CCA is due for replacement, and that goes in the report as a near-term cost. One equipment note: AGM batteries must be tested in the tester's AGM mode, or the reading is invalid.
The OBD-II Scan: Reading What the Seller Erased
Scanning for current trouble codes is the amateur version of an electronic inspection. A professional capture includes four more things: pending codes, freeze frame data, readiness monitor status, and long-term fuel trims. Pending codes are intermittent faults brewing toward a check engine light. Freeze frame preserves the conditions when a code set. Long-term fuel trims are a window into engine health; trims pulling hard in either direction point at vacuum leaks, fuel delivery, or sensor problems even with no light on.
Readiness monitors deserve special attention because they are your tamper detector. The monitors reset to not-ready whenever codes are cleared or the battery is disconnected, and they only return to ready after the vehicle completes drive cycles. So a car that arrives at your inspection with all monitors not ready is telling you someone recently cleared codes or pulled the battery. If the seller claims nothing was touched, the polite interpretation is a recent battery replacement, and the realistic one is that a check engine light was erased to get through your inspection. Either way, the correct move is to insist on a full drive cycle and a rescan before anyone calls this vehicle clean.
| Test | Pass | Fail / suspect |
|---|---|---|
| Battery voltage, engine off | About 12.6V with surface charge dissipated | Notably lower = discharged or aging battery — conductance test decides |
| Charging voltage at idle | 13.8-14.7V | Outside the window = charging system diagnosis before purchase |
| Charging voltage, full accessory load | Holds above 13.5V | Sagging toward battery voltage = alternator on borrowed time |
| Conductance / load test vs rated CCA | 75% or more of the sticker rating | Below 75% = near-term replacement cost in the report |
| AGM battery testing | Tester in AGM mode | A flooded-mode reading on an AGM is invalid in either direction |
| Readiness monitors | Complete / ready | All not-ready = codes recently cleared or battery disconnected — tamper flag, drive cycle and rescan required |
The professional scan is five things, not one: current codes, pending codes, freeze frame, readiness monitors, and long-term fuel trims — trims pulling hard past roughly ±10% point at leaks, fuel delivery, or sensors even with no light on.
- Scan before the test drive: current and pending codes both. Pending codes are intermittent faults brewing toward a light — exactly what a seller hopes stays hidden through your visit.
- Record readiness monitor status. Monitors reset when codes are cleared or the battery is disconnected, so all-not-ready is your tamper detector: insist on a full drive cycle and a rescan before anyone calls the car clean.
- Pull freeze frame for every stored code. The recorded RPM, load, temperature, and throttle position tell you the conditions to duplicate on the drive.
- Read long-term fuel trims on both banks. Trims pulling hard in either direction reveal vacuum leaks, fuel delivery, or sensor problems that have not yet turned on a light.
- Run the three-reading charging test: about 12.6V engine off, 13.8-14.7V at idle, holding above 13.5V with headlights, blower, and defroster loaded. The loaded reading is the one that predicts whether the alternator survives the buyer's first winter.
- Conductance-test the battery against its rated CCA — AGM mode if it is an AGM — and report below-75% as a near-term cost.
- Rescan after the drive. A pending code that matured during twenty minutes of driving is precisely the intermittent fault a pre-visit code-clear was hiding.
- Scanning only current trouble codes. That is the amateur version — the professional capture adds pending codes, freeze frame, readiness monitors, and fuel trims, and each one catches a class of problem the basic scan misses.
- Calling a vehicle clean with readiness monitors not ready. Not-ready monitors mean someone recently cleared codes or pulled the battery; the polite interpretation is a battery replacement, the realistic one is an erased check engine light, and either way the answer is a drive cycle and a rescan.
- Testing an AGM battery in flooded mode. The reading is invalid — it can pass a dying battery or fail a good one — and the buyer either inherits a surprise or loses negotiation leverage they were owed.
- Keeping the tamper suspicion verbal. Monitor status is a documented, objective observation: write it, note the seller's explanation, and let the buyer weigh it — a finding that never reaches paper cannot protect anyone.
Every readiness monitor shows not-ready and the seller swears nothing was touched. How do you read it, and what do you require?
Monitors only reset when codes are cleared or the battery is disconnected, and they only return to ready after completed drive cycles. The charitable read is a recent battery replacement; the realistic one is a check engine light erased to survive your inspection. Either way: full drive cycle, rescan, and no clean bill until the monitors report.
Loaded charging voltage reads 12.9V with headlights, blower, and defroster on. What does that number mean?
The alternator is not keeping up — the system is running down toward battery voltage under real load, which means the battery is quietly discharging while driving. It may pass the idle test and show no light, but it is living on borrowed time, and the report carries it as a charging system repair with a cost band.
No warning lights, no stored codes, but long-term fuel trim on bank 1 reads +18%. Is that a finding?
Yes. The ECU is adding 18% more fuel than the base map expects, which points at a vacuum leak, weak fuel delivery, or a lying sensor — a real problem that simply has not crossed the code threshold yet. Trims are exactly how a scan catches what the dash hides, and it goes in the report with a diagnosis recommendation.
Every readiness monitor shows not-ready and the seller swears nothing was touched. How do you read it, and what do you require?
Missed one? The reasoning above comes straight from the PPI exam bank, so this is the standard you will be held to.