ECU Diagnostic Codes for 4A Engines


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

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.

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) 

Malfunction Codes Indication

2-Trip detection logic:

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.

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