The $20 Scan: How to Catch Simmering Problems
Dealers can turn off the Check Engine Light, but they can't hide from 'Permanent Codes' and 'Readiness Monitors'. Here is how to check them.
Quick Answer
The seller can clear the dashboard light, but the computer remembers.
1. Get a Scanner: A $20 Bluetooth OBD2 dongle + free phone app is all you need.
2. Permanent Codes: Look for this specific field. It shows codes that cannot be cleared by a button press.
3. Readiness Monitors: If the Catalyst or EVAP monitors say "Not Ready", the seller just wiped the computer to hide a problem.
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1. The "Reset" Scam
Unscrupulous sellers will use a text tool to "Clear All Codes" right before you arrive. This turns off the Check Engine Light (CEL) for about 50 miles until the car re-detects the fault.
How to catch it:
Plug in your scanner and look at I/M Readiness (Inspection/Maintenance).
* Green/Ready: The car has been driven for days without issue. Safe.
* Red/Not Ready: The memory was just wiped. Do not buy.
2. Permanent Codes (The 2025 Standard)
Newer cars (2010+) have a "Permanent DTC" memory bank. Even if the seller clears the dashboard light, the Permanent Code remains until the car itself fixes the problem.
The Tactic: scan for "Mode $0A" or "Permanent Codes." If you see a P0420 (Catalytic Converter) here, the car needs a $1,500 repair, even if the dash light is off.
3. Live Data Red Flags
Start the engine and watch the "Live Data" stream on your phone.
* Coolant Temp: Should rise steadily to ~195°F. If it stays low (160°F), the thermostat is stuck open (hiding overheating damage).
* Fuel Trims: "Long Term Fuel Trim" should be close to 0%. If it's +15% or higher, the engine is starving for fuel or has a massive vacuum leak.
Summary
A test drive tells you how it feels. A scan tells you how it is. Never buy a car with "Not Ready" monitors unless you want to pay for the seller's repairs.
> Instant Analysis:
> MotorMigo explains what the codes on your scanner mean in plain English ("Needs Oxygen Sensor: $300") so you can use them to negotiate.
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