Common Sense Car Care

By Rick Ostien

This month’s article is about the notorious check engine light (ECM). The parts store that offers free scanning of the check engine light is hoping to sell the consumer a part to remedy the problem. However, this may or may not cure the issccue. Let us look at the way the ECM light comes on to start with.

The ECM computer is programmed to monitor the different sensors; engine, transmission, and emissions. The sensors are designed with a voltage range that is input to the ECM. If a sensor does not stay in its programmed voltage range or stays in one voltage reading too long then the ECM light comes on. The early 1980’s theory was if there was an oxygen sensor code (O2) then the sensor must need replacing. The Oxygen sensor monitors exhaust flow and changes voltage input as quick as you can click your fingers. The idea is to keep the engine from running too rich (excessive fuel) or too lean (not enough fuel). Mice love to build a nest in the air filter box. This can cause an air flow restriction which causes a rich run condition. A broken vacuum hose can cause an engine to run too lean. Both things can cause an O2 sensor to go out of range. So, you see the ECM light tells you what sensor input is having a problem, but it may not necessarily need to be replaced.

The ECM light that comes on because of an emission problem is usually the EVAP system. The system was designed to reburn fuel vapors from the fuel tank. The system is checked by the ECM to make sure there are no leaks in the system. A loose gas cap, missing gas cap, or the new capless filler neck can cause an EVAP system not to seal. Rusted parts, fuel tank, filler neck, sending unit, just about any related component under the hood can cause an EVAP problem.

The ECM light can stay on because of a problem that has not gone away (hard code). The ECM light that goes out after so many key starts is an intermittent code. The ECM will store in its memory either problem if it does not lose voltage under 9 volts or if someone clears the code. So, you can see that fixing a check engine light takes a knowledgeable technician and the equipment to do the repair correctly. We have had some customers complain about diagnostic charges. The changing of parts without correctly diagnosing the problem can cost you money and time. Many people start the conversation with we scanned the check engine light or we googled the problem and this is what I want done. This makes me recall a customer that owned a Mercedes. This vehicle had multiple codes and they wanted each part replaced that the computer coded out. I explained that we were talking about $2000 plus dollars to do this repair the way they wanted it done. We got signed permission to diagnose the problem to double check the codes that the other repair shop found. The codes we found all related to a lean running engine. Inspecting the engine showed no obvious problem. A lean running engine means that one or more cylinder is getting less fuel than the others. The most common problem causing this is a vacuum leak. We have a diagnostic tool that induces smoke into the intake manifold to look for this leak. We found a crack in the intake manifold. The diagnosis took a little more than 2 hours. The diagnostic price per hour depends on the repair facility. The average price of $100 to $180 is well spent though. In this case the total repair was around $1200. The customer saved a lot of money on parts that did not need to be replaced. It is important to note that google is a wonderful tool, but it is not the end all be all and it can end up costing you money if taken as gospel.

I hope this gives you a little insight into the ECM light. It really is important to have a trained technician diagnose and repair your vehicle. It can save you time and money. Most customers are happier saving money and having their car or truck run properly. Today’s vehicles are more complicated than ever and they really require an expert to fix them. Until next month…

Rick Ostien is the owner of Franc Motors in Willington.

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