L1 Practice Exam (Computerized Powertrain Controls Diagnosis)

19. Two technicians are discussing the cause of a random misfire diagnostic trouble code (DTC) P0304 (engine misfire detected #4 Cylinder) and P0300 set with the scan tool data shown above representing the freeze frame of the condition present when the DTC was set. Technician A says a defective or fouled sparkplug could be the cause. Technician B says a shorted ignition coil could be the case. Who is right? a. A only b. B only c. Both A and B d. Neither A nor B

- Both A and B are right. Both technicians are right. The engine control module (ECM) uses information from a crankshaft position sensor in order to determine when an engine misfire occurs and uses information from the camshaft position sensor - intake and camshaft position sensor - exhaust sensors in order to determine which cylinder is misfiring. By monitoring variations in the crankshaft rotation speed for each cylinder, the ECM is able to detect individual misfire events. If the ECM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 sets. DTCs P0301- P0304 correspond to cylinders 1-4. If the ECM is able to determine that a specific cylinder is misfiring, the DTC for that cylinder will set. Tech A is right because a defective or fouled sparkplug could cause a P0304 DTC with P0300 with AFRS (Air/Fuel Ratio Sensor) 2/1 showing HIGH voltage (4.3 volts) indicating a LEAN misfire or False Lean (see page 11 of the Composite Vehicle Type 4). This causes no burned fuel or air past bank 2 affecting AFRS 2/1. If an oxygen sensor reads low for reason besides lean mixture, it is called false lean indication. False lean indications can be contributed to: spark plug misfire or Ignition misfires. Tech B is right because, based on the scan data, a shorted ignition coil would cause DTC P0304. A defective or fouled sparkplug could cause a P0304 DTC with P0300 with AFRS 2/1 showing low voltage or FALSE LEAN indicating a misfire. Short and Long Term Fuel Trim indicates an increase of 36% to a lean Air/Fuel ratio.

20. A P0740 (torque converter clutch system problem) diagnostic trouble code (DTC) is set. The freeze frame indicated that the following parameters were present when the DTC was set. Which of the following is the MOST likely cause? a. Worn (defective) torque converter clutch b. Defective or misadjusted TP sensor c. Defective thermostat bypass valve d. Excessive EGR flow

- Worn (defective) torque converter clutch - The scan data shows the DTC and there is a 100 RPM loss from the engine speed to the turbine shaft speed indicating slippage where there should be none.

30. The OBD II (On Board Diagnostic Level 2) system on a vehicle interprets a disconnected spark plug as a misfire that could damage the three-way catalyst. On which trip will the ECM or PCM MOST likely turn on the Malfunction Indicator Lamp (MIL)? a. 1st trip b. 2nd trip c. 3rd trip d. 4th trip

-1st Trip a - It is MOST likely that on the first trip a misfire condition can occur that can damage the three way catalyst (TWC) due to overheating. Specific on board diagnostic items will cause the ECM to light up or blink the MIL, and store DTC and Freeze Frame data, even in the 1st trip; e.g., Misfire (Possible three way catalyst damage) — DTC: P0300 -P0306 will MOST likely set.

36. A DMM (Digital Multimeter) is being used to measure voltage at the ECT (engine coolant temperature) sensor. Technician A says the supply voltage to the ECT should be about 5 volts. Technician B says the voltage will increase as the engine temperature increases. Who is right? a. A only b. B only c. Both A and B d. Neither A nor B

-A only a - Technician A is right because the voltage reference to the ECT is approximately 5 volts. Technician B is wrong because the ECT is a negative coefficient thermistor, which means as the temperature increases, the resistance decreases and will pull more voltage to ground and therefore decrease (not increase) the voltage. The ECT circuit is a voltage divider network that starts as a very poor ground with about 100 kilohms of resistance and when the voltage is measured from the power supply of 5 volts to ground it will measure 5 volts. As the resistance decreases, the ground gets better and allows more voltage to pass to ground with a larger voltage drop so the sensor voltage goes down.

21. A vehicle is being diagnosed for a drivability concern and poor fuel economy and the MIL (malfunction indicator lamp) is on with the above data seen on a scan tool. Diagnostic trouble code P0131 AFRS {Air/Fuel Ratio Sensor 1/1 Circuit Low Voltage} for the air/fuel ratio sensor circuit is the only stored code. Technician A says an open circuit at Fuse #20 could be the cause. Technician B says the AFRS 1/1 Bank 1 could be contaminated. Who is right? Air/Fuel Ratio Sensor Bank 1 (AFRS 1/1) Cylinders 1-3-5 Type 4 Pg 11 1.3 volts Lambda: 0.75 Current -2000 microamps LTFT Long Term Fuel Trim Type 4 pg 24 -25% STFT Short Term Fuel Trim Type 4 pg 24 -25% a. A only b. B only c. Both A and B d. Neither A nor B

-B only b - Tech B is correct because a contaminated oxygen sensor will send a low voltage signal or no voltage signal indicating a lean condition to the ECM and will set a diagnostic trouble code (DTC) as indicated by the Type 4 Composite vehicle data shown in the table. Low voltage data indicates high oxygen at AFRS {Air/Fuel Ratio Sensor 1/1 Circuit Low Voltage}, which indicates a problem at Bank 1 oxygen sensor. Short and Long Term Fuel Trim indicates a reduction of 25% to a rich Air/Fuel ratio. Tech A is wrong because Electrical Diagram 1 of 4 on page 31 of the Composite Vehicle 4 shows that fuse #20 supplies battery voltage to all air/fuel ration sensors and the two post-catalyst oxygen sensors along with the MAF (Mass air flow) sensor, brake pedal switch, and air conditioning on/off request switch. If this circuit were open, multiple DTCs would be set, not just one and the vehicle may not run. A misfire DTC would also be present for this condition.

22. The MIL (malfunction indicator lamp) is flashing indicating a severe misfire has been detected by the engine control module. Technician A says a cracked sparkplug could be the cause. Technician B says an open at terminal "b" of ignition coil 6 could be the cause. Who is right? a. A only b. B only c. Both A and B d. Neither A nor B

-B only b - Tech B is right because Electrical Diagram 1 of 4 on page 30 of the Composite Vehicle Type 4 shows pin b of each coil is used by the engine control module to ground and unground the coil thereby creating the ignition spark. So, if open, the cylinder would misfire. Tech A is wrong because a cracked spark plug will not always cause a misfire.

26. A technician is back-probing the three (3) wires in the MAF sensor connector using a digital multimeter and a poor ground is found at the MAF sensor ground wire pin D in Electrical Diagram 2 of 3 and the diagnostic trouble code P0101 is stored. Which of the following will be the MOST likely result of this poor ground? a. Richer air/fuel mixture b. Leaner air/fuel mixture c. Low MAF sensor signal d. Low battery voltage

-Richer air/fuel mixture a - An insufficient ground at the MAF sensor could cause the MAF sensor signal voltage to be slightly higher than normal, creating a richer mixture, which, in turn, could store a diagnostic trouble code in the engine control module, causing the MIL to illuminate. See pages 11, 16, and 31 in the Composite Vehicle Type 4.

23. A technician is diagnosing diagnostic trouble code P0132 AFRS (Air/Fuel Ratio Sensor 1/1 Circuit High Voltage) on a four-cylinder engine. Technician A says an intake manifold gasket leak (vacuum leak) could be the cause. Technician B says a defective fuel pressure regulator could be the cause. Who is right? Air/Fuel Ratio Sensor Bank 1 (AFRS 1/1) Cylinders 1-3-5 Type 4 Pg 11 4.3 volts Lambda: 1.36 Current -2000 microamps LTFT Long Term Fuel Trim Type 4 pg 24 +36% STFT Short Term Fuel Trim Type 4 pg 24 +36% a. A only b. B only c. Both A and B d. Neither A nor B

-B only b - Tech B is right because a defective fuel pressure regulator could cause low fuel pressure resulting in reduced fueling on Bank 1. This will cause the AFRS 1/1 (Air/Fuel Ratio Sensor 1/1 Circuit High Voltage) to go to a high voltage setting diagnostic trouble code (DTC) P0132. Short and Long Term Fuel Trim indicates an increase of 36% to a lean Air/Fuel ratio. The fuel pressure shown is only 30 psi (review page 14 of the Type 4 book). Tech A is wrong because an intake manifold gasket leak (vacuum leak) will cause DTC P0131 AFRS 1/1 (Air/Fuel Ratio Sensor 1/1 Circuit Low Voltage) at about 1.3 volts with a Lambda of 0.75 driving rich with low voltage (see page 11 of the Composite Vehicle Type 4).

31. Technician A says the on-board self-diagnostics test mode procedures should be conducted on a cold engine. Technician B says many diagnostic trouble codes (DTC) will monitor items only when the engine control system is in closed loop. Who is right? a. A only b. B only c. Both A and B d. Neither A nor B

-B only b - Technician B is correct because many diagnostic trouble codes (DTC) will monitor items only when the engine control system is in closed loop. Closed loop operation requires the engine to reach a certain operating temperature before it can be entered. Technician A is wrong because the on-board self-diagnostics test mode procedures should be conducted on a warm engine (NOT a COLD engine as stated in the question) that is in closed loop because many DTCs only monitor in closed loop.

33. Two technicians are discussing the ECM (engine control module) reprogramming process. Technician A says the first step in the reprogramming procedure is to determine the current software needed. Technician B says in most cases using the factory scan tool or software is needed for reprogramming. Who is right? a. A only b. B only c. Both A and B d. Neither A nor B

-Both A and B c - Both technicians are correct. Technician A is right because before any reprogramming takes place you must determine what software is needed and installed. Technician B is right because reprogramming is proprietary and generally requires the factory scan tool or current software with a laptop computer and adapter.

34. Technician A says you test grounds with power applied using a DMM (digital ohmmeter) by doing a voltage drop between the ground lead and a clean common ground. Technician B says you are only allowed 0.5 volt drop across most grounds. Who is right? a. A only b. B only c. Both A and B d. Neither A nor B

-Both A and B c - Both technicians are correct. Technician A is right because to properly test a ground you do it dynamically with power in the circuit and do a voltage drop across the ground. Technician B is right because 0.5 volts is an allowable voltage drop across a ground point.

32. What component does the ECM (engine control module) or PCM (powertrain control module) monitor to determine the condition of the catalytic converter and will set a DTC (Diagnostic Trouble Code) if it fails? a. APP (accelerator pedal position) 1 and APP 2 b. Engine coolant temperature sensor c. Front and rear catalyst Temperature sensors d. Front and rear (oxygen) sensors

-Front and Rear o2 sensors. d - On vehicles equipped with OBD II, use of two or three oxygen sensors is required to monitor catalytic converter efficiency. The front 2 oxygen sensors (HOS2 1/1, HO2S 2/1) are located before the catalytic converter (upstream) and the rear oxygen sensor HO2S 1/2 is located after the converter (downstream). During closed loop fuel control, the ECM compares the signals from the 2 or 3 oxygen sensors. One important point to remember is that a rear HO2S ½ sensor is used only for diagnostics. It usually has no influence on engine operation or driveability.

35. Which of the following could cause a vehicle to run poorly after the battery was discharged or disconnected? a. PROM (programmable read only memory) failed b. EPROM (erasable programmable read only memory) failed c. Learned value has been lost from RAM (random access memory) d. Learned value has been lost from ROM (read only memory)

-Learned value has been lost from RAM (random access memory) c - When battery power is lost the learned value from the RAM (random access memory) may be lost. RAM data is electronically retained which means that it is always to some extent volatile; that is, data storage could be described as temporary - if the circuit is opened, RAM data is lost. It should be noted that most PCMs use only fully volatile RAM. In other words, when the ignition circuit is opened all RAM data is dumped. A second type of RAM is used in some PCMs. This is non-volatile RAM (NV-RAM) (or KAM-Keep alive memory) in which data is retained until either the battery is disconnected or the PCM is reset, usually by depressing a computer reset button, which temporarily opens the circuit. Codes and failure strategy (action sequence) would be written to NV-RAM and retained until reset.

18. An engine lacks power and there are no stored diagnostic trouble codes. Which of the following scan tool readings, at 2,000 RPM, is the MOST likely one to be the cause? a. AFRS (Air/Fuel Ratio Sensor) 2/1 = 2.5 Volts b. MAF (Mass Air Flow) = 2.0 gm/sec/.70 volts c. Injector pulse width = 3.3 ms d. MAP (Manifold Absolute Pressure) = 81 kPa/6 in.Hg/3.5 volts

-MAF (Mass Air Flow) = 2.0 gm/sec/.70 volts An MAF reading of only 2 grams is very low and the computer would not provide high enough fuel level for high power. The other readings are normal and a MAP sensor is only used for diagnostics on the Composite Vehicle Type 4 and no codes are set.

27. The Data Link Connector pins 4 and 16 on the Composite Vehicle Type 4 contain circuits common to all OBD-II (On-Board Diagnostics Generation 2) vehicles. What are these circuits used for? a. Power and ground b. SAE communications protocols c. ABS diagnostics d. ISO communications protocols

-Power and Ground a - Page 33 Diagram 4 of 4 on the Composite Vehicle Type 4 shows cavity 16 as the battery voltage and pin 4 is a ground. The other three choices are not found in the Composite Vehicle Type 4 booklet.

24. A vehicle with ETC (electronic throttle control) has the malfunction indicator lamp on with diagnostic trouble codes P2123 (Accelerator Pedal Position (APP) Sensor 1 Circuit High Voltage) and P2128 (Accelerator Pedal Position (APP) Sensor 2 Circuit High Voltage) stored. The vehicle runs only at fast idle. Which of the following could be the cause? a. Short to ground in APP 2 circuit b. Open in the APP 1 sensor circuit c. Short to voltage d. Ignition Switch grounded (DTC 1682)

-Short to Voltage c - Both diagnostic trouble codes are for high sensor voltage and the data stream for both sensors show high voltage. Also, when both APP sensors fail, the TAC (Throttle Actuation Control) motor defaults to 15% throttle as indicated in the data stream. Answer D is wrong because DTC 1682 is an ignition shorting problem and the engine would not run at all. Answer A is wrong because a short to ground would show zero voltage and set a different DTC (P2127 low sensor voltage). Answer B is wrong because an open in the APP 1 sensor circuit would show zero voltage and would set a P2128 high sensor voltage but not set P02123. Review page 15 of the Composite Vehicle Type 4.

28. What On-Board Diagnostics Generation 2 term applies when all enabling standards for a specific diagnostic trouble code are met? a. Freeze frame b. Trip c. Warmed up engine d. Well tuned engine

-Trip b - The term Trip is defined as when the enabling criteria for a given diagnostic code is met. This definition is found on page 26 of the Composite Vehicle Type 4.

25. Diagnostic trouble code P0101 (MASS AIR FLOW or volume airflow circuit range or performance problem) is being diagnosed. Technician A says installing the wrong air filter could be the cause. Technician B says, on an OBD II vehicle, the MAP sensor is only used for diagnostic purposes. Who is right? a. A only b. B only c. Both A and B d. Neither A nor B

c - Both technicians are right. Tech A is right because the -Both A and B MAF sensor is an air flow meter that measures the amount of air flowing in the sensor bore. The engine control module (ECM) uses the MAF sensor signal to provide the correct fuel delivery for all engine speeds and loads. A small quantity of air entering the engine indicates a deceleration or idle condition. A large quantity of air entering the engine indicates an acceleration or high load condition. The engine control module (ECM) detects that the actual measured airflow from the MAF, MAP, and throttle position sensors is not within range of the calculated airflow that is derived from the system of models for greater than 2 s. Certain aftermarket air filters may cause this DTC to set. Tech B is right because, on page 15 of the Composite Vehicle Type 4, it states that the MAP sensor in this composite is used for diagnostic purposes and barometric pressure (BARO) calculations only.

29. This question has the word LEAST in it. For this question, look for the choice that would LEAST LIKELY fit the described situation. Read the entire question carefully before choosing your answer. An EGR valve is being replaced and you need information from the vehicle identification number like the one shown in the above figure. Which of the following VIN numbers is the LEAST likely to be needed to be sure the technician receives the correct replacement part? a. Tenth digit b. eighth digit c. Sixth digit d. First digit

d - The first digit of the VIN number tells you in what -First Digit d - The first digit of the VIN number tells you in what country the vehicle was manufactured and is never needed to identify replacement parts and therefore the LEAST likely needed service information from the VIN. The 10th digit identifies the model year, which is always needed for parts ordering. The 6th digit identifies the body style and is also important for parts ordering. The 8th digit is the engine code and is always important for parts ordering.