For any serious fault finding on electronic management systems an oscilloscope of some description is essential and needn't cost the earth. Ignoring specialist automotive oscilloscopes the DIY buyer on a budget has 3 choices;
(1) Buy a second hand or basic model conventional oscilloscope cost £120.00 upwards (look in adverts in magazines such as "Everyday Practical Electronics" and "Wireless World".
(2) Buy a "Virtual Oscilloscope" which plugs into the printer port of any PC cost from £59.00 to about £350.00 (see picotech.com for details of the Pico Scope ADC range)
(3) Buy a hand held or pen style Oscilloscope cost from about £98.00 upwards (again see picotech.com for details of the £98.00 osziFOX )
I use a very basic a virtual oscilloscope which plugs into the printer port of an old Windows 3.11 486/66 PC in my garage, it is a Pico Scope ADC-10 (£59.00) which has some disadvantages for car work but the Pico Scope software is excellent. The more up market Pico Scope ADC-42 (£111.63) or dual channel storage ADC-100 (£257.33) would be better choices for those on a less restricted budget. These more expensive virtual scopes have a 1 M Ohm input impedance which allows the use of standard x10 oscilloscope probes which are ideal for measuring automotive voltages. I use an HP-2060 probe, Maplin part number AQ58N, price £19.99 including VAT.
The oziFOX pen style is sold by Pico Tech and as well as being compact and versatile can store readings for transfer and display on a PC it has received rave reviews in electronics magazines and appears remarkable value for money.
All Pico Scope equipment is also available through Maplin.