Check engine codes, in plain English
Type a trouble code (like P0420) into the box on this page and see what it means, its usual causes and symptoms, free. In the PulsCar app you can also let an OBDLink MX+ scanner read the codes from your car, and for the most common codes it tells you whether it is safe to keep driving.
Look up a code · free
Catalyst System Efficiency Below Threshold (Bank 1)
The oxygen storage capacity of the main catalytic converter on bank 1 has degraded below the OBD threshold. The downstream oxygen sensor signal increasingly mirrors the upstream sensor, indicating reduced conversion efficiency.
Usual causes
- Catalyst aged or contaminated · Common
- Downstream oxygen sensor drift · Possible
- Exhaust leak before catalyst · Less likely
- Long-term misfire damage · Less likely
Symptoms
- Check engine light on
- Failed emissions inspection
- Possibly slightly increased fuel consumption
- Sulfur (rotten egg) smell from exhaust under load
What the app does
- On this page, free: type any standard code and get its name, description, symptoms and common causes from a database of more than 10,000 codes. Pick your make to see manufacturer-specific codes.
- In the app: the same Check a code lookup, with your car profile.
- Manufacturer-specific codes for 30 makes, shown for the make in your car profile.
- Hand-written guidance for the most common codes, including whether it is safe to keep driving. For other codes the app says plainly that it cannot judge that.
- With the scanner: stored, pending and permanent codes are read every time the app connects, and while you drive it re-checks for new codes every 5 minutes and sends a notification when one appears.
How it works
- Get the app. Install the PulsCar app on iPhone or Android.
- Enter a code or connect the scanner. Type the code from your dashboard or a shop printout. Or plug the OBDLink MX+ scanner into the OBD port and connect it in the app.
- Read the explanation. See what the code means, what usually causes it, and what to do next.
Good to know
- Reading codes from the car needs the OBDLink MX+ scanner. Typing a code does not read your car. The data comes from open code databases (OBDex and others).
- The lookup on this page is free. The app’s Check a code screen and trip monitoring are part of the PulsCar membership. Without one, the app still shows your current codes, voltage and the SOS guides.
- A flashing check engine light is an emergency: pull over safely and do not keep driving.
- A code points to a system, not always to a part. Confirm repairs with a mechanic.
Questions
Do I need a scanner to look up a code?
No. Type the code in the box on this page. You need the OBDLink MX+ scanner only to read codes from your own car.
Is the lookup on this page really free?
Yes, with no sign-up. It shows what a code means, usual causes and symptoms. It is information about the code, not a measurement of your car, and it does not say whether your car is safe to drive.
Will it tell me whether I can keep driving?
For the most common codes, yes. For the rest the app explains the code and says clearly that it cannot decide that for you. When in doubt, stop.
Which scanner does it work with?
The PulsCar app works with the OBDLink MX+ scanner.
How much does it cost?
The app is part of the PulsCar membership. See the scanner and app page for current plans.
Related guides
- OBD2 Codes: Which Get a Software Fix? 1,412 Manufacturer Bulletins Analyzed
- P0420 Code in 2026: $0 to $4,500, and Five Checks Before You Buy a Catalytic Converter
- P0300 Code in 2026: Random Misfire, $0 to $5,000+, and Five Checks Before You Buy Coils
- How to Reset the Check Engine Light in 2026: Three Ways to Turn It Off, and What Each One Erases
- Best OBD2 Code Reader: Handheld, Bluetooth or Pro Tool?
- Flashing Check Engine Light + Shaking in 2026: The Light Is a Countdown, Not a Reminder
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