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Straight answers to common car questions, from battery voltage and cold starts to check-engine lights and trouble codes. Every article cites its sources.
Trouble codes & check-engine light
A trouble code, such as P0420, is what a car's computer stores when one of its self-tests finds a fault; a confirmed code turns on the check-engine light. If the light is steady and the car drives normally, get the code read within days. If it flashes, slow down and stop when it's safe.
| Code | What it means | Can I keep driving? | Article |
|---|---|---|---|
| U0100 | U0100 means another module on the car's data network, such as the transmission, brake or steering module, stopped receiving messages from the engine computer (ECM/PCM). | Treat it as urgent. GM bulletins list engine stalling, a transmission that won't shift and reduced power steering assist among the symptoms that can come with U0100 on some models. If the car has stalled, won't shift or the steering feels heavy, don't drive it; have it towed or checked where it is. | U0100 (lost communication with the engine computer): what it means |
| P0171 | P0171 means the engine computer has been adding more fuel to bank 1 than its calibrated limit allows, to correct a lean mixture, as measured by long-term fuel trim. | If the check-engine light is steady and the car runs normally, most manufacturers advise servicing as soon as conveniently possible, according to the California Air Resources Board. If the light flashes, the engine runs badly, or you smell fuel, stop driving and have it checked. | P0171 (system too lean) and fuel trims explained |
| P0101 | P0101 means the engine computer has judged the mass airflow (MAF) sensor's reading implausible when compared with its other information, even though the sensor's circuit looks intact. | If the check-engine light is steady and the car drives normally, most manufacturers advise service as soon as conveniently possible, according to the California Air Resources Board. Stop driving and get help if the engine stalls, hesitates badly, drops into reduced power, or the light flashes. | P0101 (mass airflow sensor range/performance) explained |
| P0128 | P0128 means the engine computer found that the coolant did not warm up to near the thermostat's regulating temperature, within 11.1 °C (20 °F) under California's rule, in the time the manufacturer allows. | Usually yes, if the temperature gauge stays out of the hot zone and the heater keeps the windshield clear. If the gauge climbs toward hot or an overheat warning appears, follow your owner's manual: GM says to pull off the road, stop and turn off the engine as soon as possible. | P0128 (coolant below thermostat temperature): causes and why it matters in winter |
| P0300 | P0300 means the engine computer has detected misfire in more than one cylinder, or in a cylinder it cannot reliably identify. | With a steady light and a smooth engine you can usually drive gently to a shop. A flashing light means misfire bad enough to damage the catalytic converter: reduce speed and load, and stop driving soon. | P0300 (random or multiple misfire): what it means and whether you can drive |
| P0420 | P0420 means the engine computer has judged the bank 1 catalytic converter less effective than its calibrated threshold, by comparing the oxygen sensors before and after it. | With a steady light and a smooth-running engine it is rarely an emergency, but a flashing light means a malfunction that can damage the converter: reduce driving and get it serviced promptly. | P0420: what it means, common causes, and whether you can drive |
| P0455 / P0442 | P0455 means the evaporative emission (EVAP) system found a large fuel-vapor leak, and P0442 a small one. | Usually, if the car runs normally and you cannot smell fuel. Most manufacturers advise service as soon as conveniently possible, according to the California Air Resources Board. If you smell fuel, stop, switch the engine off, get out and do not restart it until it has been checked. | P0455 and P0442 (EVAP leak): is it just the gas cap? |
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Check engine light on: can I keep driving?
Steady check engine light: usually drivable, get the code read soon. Flashing: a severe misfire, so slow down and stop when safe. When to pull over right away.
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Pending, stored and permanent trouble codes: what's the difference?
Pending codes (mode 07) are early warnings, stored codes (mode 03) turn the light on, and permanent codes (mode 0A) can't be erased by a scan tool. Here's why.
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U0100 (lost communication with the engine computer): what it means
U0100 means other modules stopped hearing from the engine computer. The causes carmakers' bulletins name, why it can mean stalling, and what to check first.
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Why does my car say "not ready" for an emissions test?
Not ready means your car's OBD self-tests haven't finished since codes were cleared or the battery was disconnected. How many are allowed, and how to set them.
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P0171 (system too lean) and fuel trims explained
P0171 means bank 1 is running lean and the engine computer has run out of fuel trim to fix it. What fuel trims are, normal ranges, and the common causes.
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P0101 (mass airflow sensor range/performance) explained
P0101 means the airflow sensor's reading doesn't agree with the engine's other sensors. What sets it, causes in carmakers' bulletins, and whether to drive.
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P0128 (coolant below thermostat temperature): causes and why it matters in winter
P0128 means your engine didn't warm up to its thermostat temperature in time. What sets it, the usual causes, and why a cold-running engine matters in winter.
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P0300 (random or multiple misfire): what it means and whether you can drive
P0300 means misfire in more than one cylinder, or in one the computer can't identify. What sets it, the common causes, and when a flashing light means stop.
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P0420: what it means, common causes, and whether you can drive
P0420 means your bank 1 catalytic converter tested below its efficiency threshold. What the car measures, the common causes, and when it is safe to drive.
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P0455 and P0442 (EVAP leak): is it just the gas cap?
P0455 and P0442 mean the EVAP system found a large or a small fuel-vapor leak. How the car tests for it, when it is the gas cap, and what else to check.
OBD2 scanners & adapters
Any standard scan tool can read a car's emission-related trouble codes, readiness and live data, with no subscription. Most low-cost adapters copy the ELM327 chip, and iPhone apps can't use the classic-Bluetooth kind. If one won't connect, turn the ignition on first. An adapter left plugged in keeps drawing current until it sleeps.
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Can you leave an OBD2 scanner plugged in all the time?
Yes while driving; parked, it depends on the adapter's sleep current. Published draws, how fast they drain a car battery, and when to unplug an OBD2 dongle.
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Can you use an OBD2 scanner without a subscription?
You don't need a subscription to read check-engine codes: the emissions data is standard by law. What scanner subscriptions add, and how to choose without one.
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Does OBD2 work on all cars?
OBD2 works on US cars and light trucks from 1996 (diesels 1997) and on Canadian ones by 2004. The exceptions: older cars, heavy trucks, imports and many EVs.
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Why won't my ELM327 adapter work with my iPhone?
Most cheap ELM327 adapters use classic Bluetooth, which iPhone apps can't reach unless the maker joined Apple's MFi program. Which ones work, and how to tell.
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Why won't my OBD2 scanner connect to the car?
OBD2 scanner not connecting to the car or ECU? Usually the ignition is off, the port's fuse is blown, the Bluetooth type is wrong or the protocol fails.
12 V battery & starting
A fully charged 12 V car battery that has rested for a few hours reads about 12.6 V (flooded) or 12.8 V (AGM); around 12.2–12.3 V means it is half charged. A click with no cranking usually means the starter isn't getting enough current. These articles cover voltage, parasitic drain, parked cars and recharging.
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Are Bluetooth car battery monitors any good?
A Bluetooth car battery monitor is a logging voltmeter: good for catching drains, low charge and charging faults, but it can't measure battery health.
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Why does my car battery keep dying?
Car battery keeps dying? It's usually worn out, undercharged by short trips, let down by the alternator, or drained while the car is off. How to tell which.
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Car won't start and just clicks: battery, starter, or something else?
A click but no crank usually points to a flat battery or a bad connection, sometimes the starter. How to tell which, jump-start safely, and when to stop trying.
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How long can a car sit before the battery dies?
How long a parked car's battery lasts depends on its parked draw, size, age and temperature. The math, what carmakers advise, and how to store a car safely.
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How long do you need to drive to recharge a car battery?
There's no reliable drive time to recharge a car battery. What battery makers say, what decides it (loads, rpm, cold, age), and why a charger works better.
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What is a normal car battery voltage?
Car battery voltage chart for flooded and AGM 12 V batteries: resting, charging and cranking voltages, how to measure them, and what each reading means.
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What is a normal parasitic battery drain, and how do you find one?
Normal parasitic drain is tens of milliamps once a car's modules sleep. How long to wait, how to measure it safely, and how to trace a drain to its circuit.
Winter (Canada)
Cold is hard on batteries: Varta says a lead-acid battery's chemical activity at 0 °C is about a quarter of its level at 20 °C, just when a cold engine is hardest to turn. Plug in a block heater at about −15 °C (5 °F) or colder, two to four hours before you drive. Written for Canadian winters.
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Why do car batteries die in cold weather?
Cold slows a car battery's chemistry as thick oil makes the engine harder to crank. What that means at −20 °C, when a battery can freeze, and how to prepare.
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When should you plug in a block heater, and for how long?
Plug in at about −15 °C (5 °F) or colder, two to four hours before you leave. What NRCan, CAA-Quebec and a maker's manual say, plus safe cord habits.
EVs
Some loss of EV battery capacity is normal, and carmakers' warranties mark the line: Hyundai's 2025 US warranty and Tesla's US warranty both use 70% of original capacity. DC fast charging can add some wear, mostly through heat. Many EVs also still have a 12-volt battery, and it can go flat. These articles explain all three.
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How much EV battery degradation is normal?
Every EV battery loses some capacity. What the Hyundai, Nissan and Tesla warranties treat as abnormal, what speeds wear up, and how to check your own battery.
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Does DC fast charging hurt an EV battery?
DC fast charging can add some battery wear, mostly through heat. What Idaho National Laboratory measured, what carmakers advise, and how to fast charge gently.
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Why do EV 12-volt batteries die, and how do you avoid it?
An EV's small 12-volt battery can leave the car dead even with a charged main pack. Why it goes flat, the warning signs, and what owner's manuals say to do.
Fleets
On light-duty vehicles from US model year 1996 on, the OBD-II port gives a fleet manager trouble codes, check engine light status, readiness and live engine data, but not tire tread or most brake wear. These articles cover that data, what idling costs, when to replace batteries, pre-trip inspections and preventive maintenance for small fleets.
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How much does idling cost a fleet?
A light-duty engine burns roughly 1 to 1.8 litres of fuel an hour at idle. How to work out what that costs your fleet each year, and how to cut it.
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What vehicle health data can OBD-II give a fleet manager?
OBD-II gives codes, check engine light status, readiness, live engine data, VIN and port voltage. It can't see tire tread, most brake wear or J1939 trucks.
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How should a small fleet schedule preventive maintenance?
Schedule each vehicle from its maker's maintenance schedule, by distance, time and engine hours, whichever comes first, and keep one record per vehicle.
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What should a pre-trip inspection cover for light-duty fleet vehicles?
Daily inspection rules cover interstate commercial vehicles from 10,001 lb (US) and trucks over 4,500 kg (Canada). Below that, a walk-around is good practice.
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When should a fleet replace vehicle batteries?
Test every fleet battery at least yearly before winter, watch batteries three to five years old closely, and replace on a failed test, not a no-start.
Rideshare & delivery drivers
The CRA treats rideshare and delivery drivers as generally self-employed: they report every payout and tip, and deduct the business share of their car costs, backed by a mileage log that the IRS also asks for. Rideshare drivers in Canada register for GST/HST from their first fare. Carmakers class delivery and taxi work as severe service.
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Do Uber, DoorDash and Skip drivers have to charge GST/HST?
Rideshare drivers in Canada register for GST/HST from their first fare; delivery-only drivers register at $30,000 over four quarters. What the CRA says.
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How do taxes work for Uber, DoorDash and Skip drivers in Canada?
Gig drivers in Canada are self-employed: report all income and tips, deduct the business share of car costs, pay both halves of CPP. The CRA's rules.
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How much wear does DoorDash or Uber put on your car?
Carmakers class taxi and delivery driving as severe service, with shorter intervals for oil, fluids and brakes. What changes, and how to put a cost on wear.
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Is DoorDash or Uber worth it after gas? How to work out your real pay
Your real pay is payouts minus fuel and wear, divided by every hour you're signed in, waiting included. The formula, fuel cost per km, and a worked example.
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What should an Uber or DoorDash mileage log include?
The CRA and the IRS ask for the same four things per work trip: date, distance, destination and purpose, kept at the time. How to log rideshare driving.
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How we write
Every Learn article is built on primary sources only: regulators, standards, carmakers' service information and battery makers' technical documents. Every claim is checked against its source before we publish, and each source is listed with the date we accessed it. Every figure comes from a cited source. Where we go beyond the source, we say so: our own arithmetic is labeled "our arithmetic", and a conclusion the source doesn't state outright is labeled "our reading" or "our inference". We use AI tools to help research and draft articles. They don't decide what is true: every claim is checked against its original source in a separate fact-check pass, and Gerrit van Rensburg, the founder, is responsible for what we publish. Spot a mistake? Email hello@tachwise.com and we will correct it.
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