The Dreaded Check Engine light

The Dreaded Check Engine light

Art’s Automotive one of the best equipped independent auto repair shops in Berkeley, the East Bay, or pretty much anywhere. Tooling and information make a huge difference in how long it takes to diagnose a problem that’s causing a check engine light and since diagnostic charge formula is time x hourly rate, speed makes a difference. Also, our hourly rate is one of the lowest in the area, so both factors in the formula are low, and you’ll likely pay less here than at other shops. Don’t take our word for it, check out Yelp, The Bay Area Consumer Checkbook, or ask some of your local friends.  We’re well regarded and that makes us really happy, so we’ll always work hard to make sure that people continue liking us.

If you know nothing at all about cars that’s OK. We can explain as much as or little as you want. If you just want to know how much it will cost and don’t care about the details, no problem. On the other hand, if you’re an enthusiast and have already pulled your trouble codes with a scanner, we can offer you assistance and give you as much information as you want. We’ll share freeze frame data, data recordings, TSBs and any diagnostic information you’re interested in. When you pay us for diagnosis, all of the information we find belongs to you,  and we’ll happily give it with you. We own a factory scanner for every make we repair: Toyota Techstream, Honda HDS, Subaru Select Monitor III, Mazda IDS, Nissan Consult III Plus, and Hyundai / Kia GDS. We also subscribe to all of the factory information websites, as well as Alldata and Identifix.

Our diagnostic fee policy

As you have probably picked up on, we charge for our diagnostic time. There are a lot of shops marketing “free check engine light diagnosis”, but what they’re usually offering is a free OBDII check. A trouble code is NOT a diagnosis. It’s simply a place to start from. If we simply replaced the part that the code points to, we’d be lucky to get 60% of our diagnosis right. Besides, what part are you supposed to replace when a P0171 (system too lean) code sets? Or a P0300 (random multiple misfire)? Not all codes point to a part. There are a few shops that take a gamble that you’ll have the repair done with them so they offer a diagnosis, then sell an overpriced repair to compensate. You probably won’t have much luck getting them to tell you what’s wrong with your car if you want to fix it yourself or take it to another shop. Our main point here is that nobody works for free. Everybody knows this intuitively, but the promise of something for nothing still draws us in and silences our rational inner voice.

We charge a minimum of one shop hour for check engine light diagnosis. This is not a charge for pulling a code. We’ll pull any OBDII code for free, so if that’s what you want, we’ll do it for you while you wait. Part of the reason for the minimum charge is to pay for our factory scanners. They have a lifespan of about 3 years and need to updated frequently, and all of them require a software subscription, which costs us about $12,000 per year. Factory scanners provide FAR more information than the OBDII specification requires. We will spend up to an hour looking at your car. This includes research, disassembly, manual component testing, or whatever is necessary to find the problem. In the event we are unable to find the problem within an hour (which is very rare), we will call you at the end of the hour and explain what’s been tested, what the results were, and what we are recommending be done next. 95% of the time we’ll simply call you at the end of the of the hour and tell you what’s wrong and how much it will be to fix.

Some common problems and explanations

Misfire codes: P0300, P0301, P0302, P0303, P0304, P0305, P0306. These codes set when there is a misfire. What is a misfire? A misfire is when the air/fuel mixture drawn into the combustion chamber does not ignite. This can happen if the air/fuel mixture is too lean (not enough fuel), if there is a problem with the spark plug or ignition system, or if there is an engine mechanical problem causing poor sealing of the combustion chamber.

How does the computer know there’s a misfire? There is a reluctor pressed or keyed onto the crankshaft snout. This star-shaped magnet spins past a pickup coil. A pickup coil is a coil of wire wrapped around an iron core. When a point on the magnetized reluctor passes the pickup coil, it generates a voltage. This voltage will be a sine wave; the voltage climbs as the reluctor tip nears the pickup coil and dips at the reluctor moves away from the pickup coil. The computer looks at the time between each pulse created by the reluctor. The pulses should be equally spaced if there is not a misfire. If a cylinder misfires, the crankshaft speed will slow for a short time because of the misfire. Imagine sitting in a pedicab (sort of like a rikshaw with a bicycle attached). If your driver is standing on the pedals while climbing a hill, you’ll feel the pulses with each pedal down-stroke. If his foot slips off the pedal at the top of the down-stroke, you’ll feel the missing pulse. This is what the ECM is doing, just electronically.

 Catalyst codes: P0420, P0430. These codes set when the ECM judges that the catalyst has degraded. What’s a catalyst? It’s a coated ceramic element sitting in the exhaust flow. It looks a bit like a bunch of small straws. The element is dipped in platinum, palladium, and rhodium. When the exhaust gases flow through the element, contact with the catalysts causes oxidation and reduction reactions. Nitrogen oxides (NOx) are reduced to Nitrogen (N) and Oxygen (O). Carbon Monoxide (CO) is oxidized to Carbon Dioxide (CO2) Hydrocarbons (HC) are oxidized to water (H2O) and Carbon Dioxide (CO2). In other words, the nasty gases  coming out of your exhaust pipe are converted to water and carbon dioxide.

So, how does the ECM know if the catalyst is degraded? The ECM monitors the front air/fuel ratio sensor and the rear oxygen sensor. The ECM will actively alter the air fuel mixture and observe the response of the rear O2 sensor on newer cars. Older cars use a passive technique and count the rear O2 sensor cycles. Here’s the thing, a P0420 can be caused by a lot of things besides a degraded converter. However, replacing the converter will often “fix” the check engine light, even though it is not the primary cause of the light. Air fuel mixture is extremely important. The catalyst only works in a narrow window of air fuel ratios so checking fuel trims or measuring the exhaust gas content to check for ratio is an important check. Exhaust leaks also cause the catalyst to be inefficient because if air leaks into the exhaust, it affects the reduction reaction. Misfires are another potential cause of a P0420 that is not the converter.

EVAP codes: P0440, P0442, P0446, P0455. The EVAP system is designed to capture and store fuel vapor until it can be burned by the engine. Gasoline is volatile, which means that it will evaporate at normal temperatures. The fuel tank must be vented, otherwise pressure would build as it heated and a vacuum would be created when it cooled. This could damage the tank. Click here to see a video of a railroad tanker collapsing under vacuum. The same thing would happen to the fuel tank if it was not vented. The problem with venting the tank is that the gasoline would evaporate, which would put hydrocarbons (HC) into the atmosphere. This is one of the components of the orange smog you see in the Bay Area on hot days. Instead of venting straight to atmosphere, the tank is vented through a charcoal canister which adsorbs the HCs so they are not released into the air. Later, when the car is running, air is pulled through the charcoal canister and into the engine, so the HCs are stripped from the charcoal and burned in the engine.

EVAP codes set when the ECM (engine control module) performs a self-test of the EVAP system and the test fails. The ECM self-tests are also known as “monitors”. The EVAP self-test is known as “the EVAP monitor”. There are many types of EVAP systems and self-tests, but nearly all systems apply a vacuum to the fuel tank and then monitor the tank pressure with a sensor. Usually this sensor is a 3-wire pressure transducer. Two wires power the unit one with a regulated voltage source, the other connected to a sensor ground. The third wire is voltage that varies with pressure. The pressure sensor very accurately measures a very small pressure range.


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