Check Engine Light (CEL)

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Some engine problems will cause the orange engine tell-tale (Check Engine Light / CEL ) to be illuminated by the ECU, there are several methods to determine what the problem is.


Trouble codes may either be intermittent or hard (that is, the fault may be temporary and then reset, or it may be persistent).

Temporary problems cause the check engine light (hereafter called the light) to illuminate and then, when the fault is fixed, the light will go out, usually around 10 seconds later. Fault codes will be stored in the ECU for the next 50 engine starts.

Certain faults will cause the ECU to limit the max engine RPM to 4000 to prevent engine damage.

Accessing stored fault codes

There are a couple of methods available to read the fault codes out of the ECU:

  • Using ElanScan
  • The paper-clip method

Using ElanScan

ElanScan can read out the fault codes from the ECU; check the sensors & flags list on the left of the ElanScan window.

The Paper Clip Method

You will need a paper clip or small electrical wire to do the deed! Bend the paper clip into a "U" shape and read on...

Below is a crude drawing of the end on appearance of the ALDL.


From the Lotus Service Manual:

If the diagnostic terminal (B) of the ALDL connector is grounded to terminal (A) with the ignition on and the engine stopped, the system will enter the diagnostic mode.

Since the end of the ALDL is NOT marked with little letters please note that the A pin is the Black wire and the B pin is the Black/White wire !

Codes are displayed by the light quickly flashing the first digit of the two-digit number, after a short pause the second digit is similarly flashed. For example, code 12 would be one flash—pause—two flashes.

Be ready to count the flashes to figure out your error code!

First a code 12 will be flashed by the light to indicate that the diagnostic mode is active. If no trouble codes are stored, code 12 will continue to be displayed in succession until the diagnostic terminal is ungrounded. If one or more codes are stored, code 12 will flash THREE times, after a short pause the first trouble code will be flashed THREE times, then the next code, and so on while further stored codes remain, finally code 12 is flashed again to complete the cycle.

The Video

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Fault Codes

Engine Fault Codes
Code Fault
12 None
13 O2 sensor (or circuit) malfunction -- see CommonParts replacement
14 High Coolant Temperature (relative to other measurements)
15 Low Coolant Temperature (relative to other measurements) -- see CTS write-up
21 Throttle Position Sensor (Signal Voltage High)
22 Throttle Position Sensor (Signal Voltage Low)
23 Manifold Air Temperature Low (relative)
24 Vehicle Speed Sensor
25 Manifold Air Temperature High (relative)
31 Manifold Absolute Pressure (Turbo Overboost)
33 Manifold Absolute Pressure Sensor (High voltage / Low vacuum)
34 Manifold Absolute Pressure Sensor (Low voltage / High vacuum)
35 IAC error IAC motor not controlling the engine to the desired speed
41 Cam Angle Sensor (Hall Effect Sensor) -- see CAS write-up
42 Electronic Spark Timing circuit
43 Electronic Spark Control (Knock Sensor Circuit)
44 Lean exhaust
45 Rich exhaust
51 ECU Fault ($$$OUCH$$$--get a new ECU)
53 Over voltage (system voltage high)
54 CO-Potentiometer circuit
65 Fuel injector driver circuit (or output) problem

Intermittent Faults