Motorheads D.I.Y. Garage

Tell me about that “Check Engine Light”. 

The automotive service industry calls the Check Engine light on your dash an “MIL” or Malfunction Indicator Light. It shows three different types of signals. Occasional flashes show momentary malfunctions. It stays on if the problem is of a more serious nature, affecting the emissions output or safety of the vehicle. A constantly flashing MIL is a sign of a major problem which can cause serious damage if the engine is not stopped immediately. In all cases a “freeze frame” of all sensor readings at the time is recorded in the central computer of the vehicle.

Hard failure signals caused by serious problems will cause the MIL to stay on any time the car is running until the problem is repaired and the MIL reset. Intermittent failures cause the MIL to light momentarily and they often go out before the problem is located. The freeze frame of the car’s condition captured in the computer at the time of the malfunction can be very valuable in diagnosing these intermittent problems. However, in some cases if the car completes three driving cycles without a re-occurrence of the problem, the freeze frame will be erased. On-board diagnostic systems are more sophisticated than ever. OBD2, a new standard introduced in the mid-’90s, provides almost complete engine control while also monitoring chassis, body and accessory devices.

Testing your OBD2

Before You Begin

Since all vehicles are not the same please refer to the manual supplied with the OBD2 code reader for vehicle specific instructions.

Is your vehicle compliant?

All cars and light trucks manufactured since late 1995 should be OBD2 compliant. Two factors will show if your vehicle is definitely OBD2 equipped:

• There will be a note on a sticker or nameplate under the hood: “OBD2 compliant”

• There will be an OBD2 connector as shown below.

Where is the connector located?

The connector must be located within three feet of the driver and must not require any tools to be revealed. Look under the dash and behind ashtrays.

Important info about OBD2

• Can diagnose/check engine problems in all CAN and OBD2 vehicles (1996 and newer) and many of the most popular OBD1 (1981 to 1995) vehicles
• Features unique patented all-in-one screen display and LED display for quick emissions check
• Automatic refresh updates data every 30 seconds when connected to the vehicle – an easy way to verify repair completion
• Code definition can be displayed in English, French and Spanish
• Can be Flash updated with a standard Windows® PC
• Features memory/battery backup for off-car review and analysis

Connecting

Connecting the OBD2 Code Reader is as simple as locating the DLC connector. The DLC connector is usually found under the driver side (left side) of the dash. The cable attaches only one.

Reading the LCD Display

Before you start testing, take a good look at the LCD display. The illustration above explains what the various icons mean and how the DTC (Diagnostic Trouble Code) will be displayed. The unit is self-powered by three-AAA batteries and will let you know when they are running low. When connected/receiving power from the vehicle, the CAR Icon will display.

The OB2 code reader is a very simple tool to use. Once it is connected, turn the power on. Once the ignition is switched to the on position, the code reader is checking the system for any stored DTCs.

The meaning of each DTC can be found in the manual supplied with your code reader.

Example: the code reader displays a DTC PO117, you can look these up on the internet  for definitions, possible causes and solutions.

P0117 is ECT Circuit Malfunction

The ECT (Engine coolant temperature) sensor is a thermistor located in the engine block or other coolant passage. It changes resistance with changes in temperature of the coolant that it’s in contact with. It’s usually a two wire sensor. One wire is a 5 volt reference from PCM (Powertrain Control Module) and the other is a ground supplied from PCM.

As the temperature of the coolant changes, the resistance of the sensor changes. When the engine is cold, the resistance is high. When the engine is warm, the resistance is low. If the PCM detects a signal voltage lower than the normal operating range of the sensor then P0117 code will set.

Potential symptoms include:

• MIL (Malfunction Indicator Lamp) illumination
• Poor fuel economy
• Poor drivability
• Engine may run rough or blow black smoke out the tailpipe
• May not idle
• May start and then die

Potential causes of the P0117 code include:

• Bad ECT sensor
• Short to ground on ECT signal circuit
• Faulty or damaged connectors
• Wiring harness damaged
• Loose terminals at ECT or PCM
• POSSIBLY an overheated engine
• Bad PCM

Since this code is for an abnormally low signal to the PCM from the ECT, the PCM saw a excessively “hot” condition in the engine coolant. This could be due to faulty ECT sensor or wiring, but it could, conceivably, be caused by an overheated engine.So if your engine was overheated diagnose that first.

Having said that here are the possible solutions:

Using a scan tool, with KOEO (Key on engine off) check the ECT reading on the display. On a cold engine the ECT reading should match the IAT (Intake Air Temperature) sensor reading. If it doesn’t, replace the ECT sensor.

1. If the ECT reading is showing an excessively hot reading, for example more than 260 deg. F, then unplug the ECT sensor. This should cause the ECT reading to drop to the extreme cold reading (around -30 deg. F or so). If it does, it’s internally shorted, replace the sensor. If it doesn’t change the reading, then check for a short to ground on the signal circuit of the ECT wiring. It could be that the two ECT wires are shorted to each other. Look for any chafing or melted wiring. Repair as necessary.

a. If you can’t find any wiring problems and the ECT reading doesn’t drop to the extreme coldest when unplugged, then, check for voltage coming out of the PCM on the signal wire’s pin at the PCM connector. If there isn’t any voltage, or it is low, then the PCM may be bad. NOTE: On some models it is possible for the 5 Volt reference signal to be temporarily shorted. This can happen if an engine sensor internally shorts the 5 Volt reference. Since the 5 Volt reference is a “shared” circuit on many models, this will cause it to be abnormally low. Usually, though this will be accompanied by several other sensor codes. If you suspect that may be the case, unplug each sensor until the 5 Volt reference reappears. The last sensor unplugged is the offending sensor. Replace and recheck the signal wire from the PCM connector

2. If the scan tool ECT reading seems normal at this time then the problem may be intermittent. Use a “wiggle” test to manipulate the wiring harness and connectors while watching the ECT reading on the scan tool. Repair any wiring or connectors that are loose or corroded. You could check the Freeze frame data if your scan tool has that function. It will show the ECT reading when the failure occurred. If it shows the reading to be at the hottest extreme then replace the ECT sensor and see if the code reappears.

Other ECT sensor and circuit related DTC’s are: P0115, P0116, P0118, P0119, P0125, and P0128

Once you know what the problem is, you have to determine what’s causing it. In this case the intake manifold gasket was faulty. Once the repairs have been made, it is time to clear the DTC and check the system to verify the repair.

Clearing the Diagnostic Trouble Code (DTC)

Clearing the DTC is very simple. Press the ERASE button on the code reader. A message will display on the screen; “SURE” for conformation. If you wish to clear the DTC press and hold down the ERASE button again until a message appears on the screen “DONE”. If you did not wish to clear the codes and hit “Erase” by mistake at the screen message “SURE” simply press the link button to return without erasing any DTCs.