I’m going to tell you something that most automotive writers wouldn’t admit.
I ignored a check engine light for three months. Not on a customer’s car. On my own.
I know what you’re thinking – the guy who writes about cars for a living, who tells people to get codes read immediately, who knows exactly what that amber light means. Yes. That guy. Me.
I have a reason. It wasn’t a good one, but I’ll give it to you anyway, because the honest version of this story is more useful than a sanitized one.
How It Started
The light came on a Tuesday in late October. I was in the middle of three other projects, running behind on everything, and the car was driving completely normally. No hesitation, no rough idle, no change in fuel economy that I’d noticed. The light was steady — not flashing, which I knew meant it wasn’t an active misfire destroying my catalytic converter in real time.
I told myself I’d read the code that weekend.
That weekend came and went. The code reader was in my other car. Then it rained. Then I got busy. You know how this goes. The light stayed on, the car kept driving normally, and the urgency I felt on Tuesday faded into background noise by Thursday.
Three months later, I finally read the code.
What I found is the reason I’m writing this article.
The Code: P0420
P0420. Catalytic converter efficiency below threshold, Bank 1.
I want to be clear about what this code means and what I assumed it meant — because those two things were not the same, and the gap between them is exactly where my mistake lived.
What I assumed: the catalytic converter was failing. The converter is an expensive repair — $400 to $1,500 depending on the vehicle — and since the car was running fine, I mentally filed it under “deal with this later.” I’ve seen plenty of P0420 codes that turned out to be the converter at end of life. The car was high mileage. It seemed like the obvious explanation.
What I should have done: read the code, then diagnosed the actual cause instead of assuming I already knew it.
Because P0420 has multiple possible causes. A failing catalytic converter is one of them. But so is a faulty downstream oxygen sensor. And so is an exhaust leak upstream of the converter. And so — critically — is an engine running rich, dumping unburned fuel into the exhaust stream, which both damages the catalytic converter and indicates a separate underlying problem.
I skipped the diagnosis step. I assumed the answer and decided to deal with the expensive answer later.
What Three Months of Ignoring It Actually Cost Me
When I finally put the car on the lift and did a proper diagnosis, here’s what I found.
The actual cause of the P0420: A failing coolant temperature sensor was feeding the ECU incorrect temperature data, causing the engine to run slightly rich when the coolant sensor reported a lower temperature than actual. The engine thought it was cold when it wasn’t. Rich running condition. Unburned fuel entering the exhaust. Catalyst efficiency dropping.
The coolant temperature sensor: $18 part. Fifteen-minute replacement. This was the root cause.
The catalytic converter: Three months of an engine running rich — dumping excess unburned fuel through the exhaust system — had accelerated the degradation of a converter that was already past its peak but had years of life remaining. When I tested converter efficiency before and after fixing the sensor, the difference was measurable. The converter was damaged. Not destroyed — but measurably worse than it would have been if I’d caught the sensor failure in October instead of January.
The downstream oxygen sensor: Also degraded, likely from the same rich condition. Another $65 part I now needed.
Total repair cost: $18 sensor + $65 oxygen sensor + $340 for a replacement catalytic converter that I could have avoided for at least another two to three years if I’d fixed the $18 sensor in October = $423.
If I had read the code in October and diagnosed correctly: $18.
The math is not subtle.
The Part That Bothers Me Most
I want to sit with the oxygen sensor for a moment because this is the part of the story I find most instructive.
The downstream oxygen sensor is what the ECU uses to measure catalytic converter efficiency. When the converter is working properly, the downstream sensor shows a relatively stable voltage because the converter is processing the exhaust evenly. When efficiency drops, the downstream sensor starts mimicking the upstream sensor — fluctuating, which triggers the P0420.
But the downstream sensor also degrades on its own timeline. A sensor running in an exhaust stream that’s consistently contaminated with excess fuel – which is what a rich-running engine produces — degrades faster than one in a properly combusted exhaust stream.
So the sequence was: bad coolant temp sensor → rich condition → damaged catalytic converter → degraded oxygen sensor → P0420 code I ignored → continued rich condition → more damage → three months of compounding.
Every week I didn’t fix the $18 sensor was another week of excess fuel washing through components that weren’t designed to handle it.
What the Car Was Telling Me That I Wasn’t Listening To
Here’s the thing about driving a car with a P0420 for three months: it felt fine. The rich condition caused by the failed coolant sensor was mild enough that I didn’t feel it in normal driving. Fuel economy dropped slightly – I noticed the tank seemed to empty a bit faster but attributed it to the colder weather and more short trips.
That slight fuel economy drop was the only symptom available to me without reading the code. And I explained it away.
This is how gradual damage works. It doesn’t announce itself dramatically. It accumulates quietly while you find reasons not to deal with the amber light on your dashboard.
The check engine light is not a “something is definitely broken right now” indicator. It’s a “the computer has detected something outside normal parameters” indicator. That could be a loose gas cap. It could be an $18 sensor. It could be the beginning of a chain of events that costs you $400 if you act now and $1,500 if you wait.
You cannot know which one it is without reading the code. And you cannot know the actual cause without diagnosing past the code.
What I Do Differently Now
This experience changed one specific habit. I now keep an OBD2 scanner in every vehicle I own. Not in my other car, not in my toolbox at home. In the glovebox of the vehicle. Plugged in takes sixty seconds. Code read takes another thirty. I have no excuse to wait a weekend.
If you don’t own an OBD2 scanner, every AutoZone, O’Reilly, and Advance Auto Parts location will read your codes for free while you wait. There is no logistical barrier to knowing what your check engine light is trying to tell you within 24 hours of it coming on.
The barrier is the same one I had: the assumption that since the car feels fine, the problem can wait.
Some problems can wait. Some problems get more expensive every week you wait. Without reading the code, you have no way to know which situation you’re in.
The One Thing I Want You to Take From This
I write about automotive maintenance because I think informed car owners make better decisions and spend less money over time.
This story is me being an uninformed car owner for three months. The decision I made — or rather, the decision I kept not making — cost me $405 that I didn’t need to spend.
Read the code. Diagnose the cause. Fix the cheapest possible solution first and verify the result. Don’t assume you know what the code means before you’ve looked at what’s actually happening.
The check engine light is not your enemy. It’s the only early warning system your car has. I ignored mine for three months and paid for it in a way that was entirely, completely, avoidably my fault.
Don’t repeat my mistake.
Written by Emran Russell, automotive performance parts specialist and founder of AutomobileBee.com. Emran has hands-on experience diagnosing and repairing vehicles across a wide range of makes and applications — including, occasionally, his own.
Related Articles:
- What Does the Check Engine Light Actually Mean? A Car Owner’s Plain-English Guide
- OBD2 Scanner Buying Guide: What Every Car Owner Should Know Before Buying One
- The Honest Truth About Cheap vs Expensive Car Parts