ECU Diagnostic Codes for 4A Engines

The following codes apply to all Toyota 4A series engines.  

  • 4A-GE

  • 4A-GZE

  • 4A-GE 20V

Some codes may not exist in earlier or non-applicable engine ECU's  but all codes are common between 4A and many other Toyota ECU.   It is essential that you do have the proper manuals and skills necessary to perform diagnostic and repairs.  This diagram is to assist those who are qualified in obtaining some information out of the ECU on cars equipped with factory units.

Diagnosis System: To obtain output, proceed as follows.

Initial conditions must be met for codes to appear in normal output.

  • 1. Battery Voltage must be 11V or more

  • 2. Throttle valves must be fully closed (on newer engines)

  • 3. Accessories switched off

  • 4. Engine at operating temperature

Turn the ignition switch ON. (Do not start engine)

Short terminals TE1 and E1 of the check connector of the diagnosis connector cluster.

Diagnostic Codes:

Normal Operation: (no malfunction)

  • the light will alternatively blink ON and OFF at 0.26 second intervals.

Malfunction Codes Indication

  • In the event of a registered malfunction, the light will blink every 0.52 second.  The first series of blinks will equal the 1st digit of a 2 digit code and after a 1.5second pause, the second series of blinks will indicate the 2nd digit.

  • If there are 2 or more codes, there will be a 2.5 seconds pause between each signaled codes.

  • After all codes have flashed, there will be a 4.5 second interval and the codes will repeat until TE1 and E1 has been disconnected.

2-Trip detection logic:

  • Diagnostic Codes 20-29 are usually indicated with 2 -trip logic.  This means that the ECU will need two repeated engine operational cycles, with the same problem for the code to be indicated, minimizing the false codes.  The ignition must be switched OFF between the 2 successive engine operations.

Test Mode: (1989 and later ECU, with variation in models))

In this mode, the ECU will have increased sensing ability for malfunction detection.  It can also detect starter signal circuit, and air conditioning components.  The same codes also will be detected as those of the normal modes.

  • In the same manner and conditions as the normal diagnostic, short the TE2 and E1 of the diagnostic connector.  Turn ignition ON to begin diagnosis in TEST mode.

  • To check that the TEST mode is operational, confirm that the warning light flashes with ignition in ON position.

  • Start the engine and drive the vehicle at or faster than 10km/h (6mph).  If the speed fails to reach this the ECU will falsely report that the starter and speed sensor is malfunctioning.  (code 41 and 42)

  • After test drive, short connectors TE1 and E1

  • Read diagnostic codes.

  • be sure to Remove SST from check connector when finished.

Warning: Test mode will not start if terminals are shorted after ignition is turned ON. Code 51 (AC compressor switch signal) is erratic by nature and will sometimes come on without malfunction.

The following are some common codes for easy troubleshooting, there are others which I will add at a later time when my list is more complete.

Code Signal Diagnosis Common Trouble Area
12 RPM Signal
  • No G or NE signal present for 2 seconds during cranking
  • Open G circuit
  • Open, short in G or NE circuit. 
  • Distributor
  • ECU 
13 RPM signal
  • No NE signal for 50msec at 1000rpm.
  • Same as Above
14 Ignition
  • No IGF signal to ECU for 4 consecutive IGT signal during engine running 
  • Open/short in IGF/IGT signal from igniter to ECU
  • Igniter malfunction
  • ECU
21 O2 sensor
  • Open/short in heater circuit of oxygen sensor for 0.5 seconds 
  • Amplitude of O2 sensor reduced to 0.35-0.70V continuously for more than 60 seconds
  • O2 sensor
  • vacuum sensor
  • ECU 


22 H2O temperature sensor
  • no signal at THW
  • water temperature sensor
  • ECU
24 Intake Air Temp sensor
  • no signal at THA
  • Intake air temp sensor
  • ECU
25 A/F Lean 
  • O2 sensor output in less than 0.45 volts for at least 90 seconds when O2 sensor is warmed and 1500 rpm or above.
  • open injector circuit
  • lack of fuel pressure 
  • Ignition system
  • Vacuum sensor
  • O2 sensor
  • ECU
31 Vacuum sensor 
  • No signal at Vacuum sensor (PIM)
  • Vacuum sensor to ECU
33  ISCV (Idle speed control valve)
  • Open or short in ISCV circuit
  • ISCV Valve
41 Throttle Position sensor
  • No signal for 0.5 seconds or more at ignition ON. (VTA)
  • TPS
  • ECU
42 Vehicle Speed Sensor
  • No speed sensor signal to ECU when loaded driving condition with engine speeds 2000 to 5000 rpm.  for 8 seconds or more.
  • VSS
  • ECU
43 Starter 
  • No starter signal to ECU when cranking in TEST mode.  Terminal STA on ECU
  • starter signal circuit
  • starter relay
  • ignition switch
52 #1 Knock sensor 
  • no KNK1 signal to ECU
  • knock sensor (looseness)
  • malfunctioning knk sensor
53 #2 Knock sensor
  • no KNK2 signal to ECU
  • same as above
51 Switch condition signal
  • Displayed when A/C is on when TE1 and E1 connected in test mode
  • A/C switch
  • Accelerator pedal, cable
  • ECU