Vehicle Tips & FAQs

How Much Does a Diagnosis Cost?

This post is a little more technical than most. I've included a glossary below should you like more information about a term.

What does it cost to perform a check engine light diagnosis? Repairpal says between $88-111.

What does it cost to skip a diagnosis? It could be a lot.

Let's give an example: if the check engine light on your 2008 Mini Cooper comes on and you're a do-it-yourselfer, you immediately grab your trusty scan tool, or you head to your local parts store. The diagnostic trouble code you find stored in the ECU is P0420 which is the universal code for catalyst inefficiency (bank 1 if the engine has two banks). This means the catalytic converter is not reducing emissions the way that it's designed to do.

What's a catalytic converter cost? Well, almost $1100 for a genuine one on and installation can cost up to $1700 if you decide not to do it yourself. Even an aftermarket part will cost several hundred dollars. If you're an experienced shade tree mechanic, you'll want to verify your diagnosis by examining the exhaust. You don't hear any leaks and the wiring on the oxygen sensors looks ok. So, you go ahead and replace the catalytic converter. But did you really think of everything?

Did you know that an engine coolant temperature sensor can be to blame for a P0420 code? But why? This sensor is nowhere near the catalytic converter or even the exhaust for that matter.

Vehicles are designed to inject more fuel into an engine when the vehicle is starting cold. If the ECU thinks it's cold even when the engine is not, the catalytic converter could be overwhelmed by unburned fuel which causes the oxygen sensor to think it's performing below its threshold efficiency.

This is a rare scenario occurring in probably 5% of cases or less. But do you want to be one of the 5% it happens to? The reason that parts stores can get away with recommending parts for a specific code is that they're right 60-75% of the time. Why waste time and money replacing unnecessary parts?

The following are some unusual diagnoses for their symptoms. Hopefully it's interesting to you.

Battery going dead -- Radio has internal short

Car has trouble starting -- Mouse nest inside air filter box or exhaust

Air blowing but no heat -- Conductive engine coolant causing pinhole leaks

Car dies but has fuel, spark, and compression -- Stale gas

Headlights intermittently switch off -- Failing parking brake switch

Engine keeps running after shutoff -- Cracked fuel pump


ECU:sometimes synonymous with PCM or ECM. Computer or control unit that adjusts engine running conditions

Bank:a row of pistons. E.g. a V6 engine has two banks but an I6 or straight-6 engine has one bank.

Catalytic Converter: a housing in your exhaust that looks like a muffler but is designed to reduce emissions. It “catalyzes” the process that breaks down harmful emissions into harmless ones.

Oxygen Sensor: a sensor mounted in your exhaust system that measures ratio between fuel byproducts and air.